[mod] replace /help by /info pages and include pages in project docs

This patch implements a bolierplate to share content from info-pages of the
SearXNG instance (URL /info) with the project documentation (path /docs/user).

The info pages are using Markdown (CommonMark), to include them in the project
documentation (reST) the myst-parser [1] is used in the Sphinx-doc build chain.

If base_url is known (defined in settings.yml) links to the instance are also
inserted into the project documentation::

    searxng_extra/docs_prebuild

[1] https://www.sphinx-doc.org/en/master/usage/markdown.html

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/infopage/__init__.py

@@ -0,0 +1,191 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# lint: pylint
+# pyright: basic
+"""Render SearXNG instance documentation.
+
+Usage in a Flask app route:
+
+.. code:: python
+
+  from searx import infopage
+
+  _INFO_PAGES = infopage.InfoPageSet(infopage.MistletoePage)
+
+  @app.route('/info/<pagename>', methods=['GET'])
+  def info(pagename):
+
+      locale = request.preferences.get_value('locale')
+      page = _INFO_PAGES.get_page(pagename, locale)
+
+"""
+
+__all__ = ['InfoPage', 'MistletoePage', 'InfoPageSet']
+
+import os.path
+import logging
+from functools import cached_property
+import typing
+
+import urllib.parse
+import jinja2
+from flask.helpers import url_for
+import mistletoe
+
+from .. import get_setting
+from ..version import GIT_URL
+
+logger = logging.getLogger('doc')
+
+
+class InfoPage:
+    """A page of the :py:obj:`online documentation <InfoPageSet>`."""
+
+    def __init__(self, fname, base_url=None):
+        self.fname = fname
+        self.base_url = base_url
+
+    @cached_property
+    def raw_content(self):
+        """Raw content of the page (without any jinja rendering)"""
+        with open(self.fname, 'r', encoding='utf-8') as f:
+            return f.read()
+
+    @cached_property
+    def content(self):
+        """Content of the page (rendered in a Jinja conntext)"""
+        ctx = self.get_ctx()
+        template = jinja2.Environment().from_string(self.raw_content)
+        return template.render(**ctx)
+
+    @cached_property
+    def title(self):
+        """Title of the content (without any markup)"""
+        t = ""
+        for l in self.raw_content.split('\n'):
+            if l.startswith('# '):
+                t = l.strip('# ')
+        return t
+
+    def get_ctx(self):  # pylint: disable=no-self-use
+        """Jinja context to render :py:obj:`InfoPage.content`"""
+
+        def _md_link(name, url):
+            url = url_for(url)
+            if self.base_url:
+                url = self.base_url + url
+            return "[%s](%s)" % (name, url)
+
+        def _md_search(query):
+            url = '%s?q=%s' % (url_for('search'), urllib.parse.quote(query))
+            if self.base_url:
+                url = self.base_url + url
+            return '[%s](%s)' % (query, url)
+
+        ctx = {}
+        ctx['GIT_URL'] = GIT_URL
+        ctx['get_setting'] = get_setting
+        ctx['link'] = _md_link
+        ctx['search'] = _md_search
+
+        return ctx
+
+    def render(self):
+        """Render / return content"""
+        return self.content
+
+
+class MistletoePage(InfoPage):
+    """A HTML page of the :py:obj:`online documentation <InfoPageSet>`."""
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
+    @cached_property
+    def html(self):
+        """HTML representation of this page"""
+        return self.render()
+
+    def render(self):
+        """Render Markdown (CommonMark_) to HTML by using mistletoe_.
+
+        .. _CommonMark: https://commonmark.org/
+        .. _mistletoe: https://github.com/miyuchina/mistletoe
+
+        """
+        return mistletoe.markdown(self.content)
+
+
+_INFO_FOLDER = os.path.abspath(os.path.join(os.path.dirname(__file__), '..', 'info'))
+
+
+class InfoPageSet:  # pylint: disable=too-few-public-methods
+    """Cached rendering of the online documentation a SearXNG instance has.
+
+    :param page_class: render online documentation by :py:obj:`InfoPage` parser.
+    :type page_class: :py:obj:`InfoPage`
+    """
+
+    def __init__(self, page_class: typing.Type[InfoPage], base_url=None):
+        self.page_class = page_class
+        self.base_url = base_url
+        self.CACHE: typing.Dict[tuple, InfoPage] = {}
+
+        # future: could be set from settings.xml
+
+        self.folder: str = _INFO_FOLDER
+        """location of the Markdwon files"""
+
+        self.i18n_origin: str = 'en'
+        """default language"""
+
+        self.l10n: typing.List = [
+            'en',
+        ]
+        """list of supported languages (aka locales)"""
+
+        self.toc: typing.List = [
+            'search-syntax',
+            'about',
+        ]
+        """list of articles in the online documentation"""
+
+    def get_page(self, pagename: str, locale: typing.Optional[str] = None):
+        """Return ``pagename`` instance of :py:obj:`InfoPage`
+
+        :param pagename: name of the page, a value from :py:obj:`InfoPageSet.toc`
+        :type pagename: str
+
+        :param locale: language of the page, e.g. ``en``, ``zh_Hans_CN``
+                       (default: :py:obj:`InfoPageSet.i18n_origin`)
+        :type locale: str
+
+        """
+        if pagename not in self.toc:
+            return None
+        if locale is not None and locale not in self.l10n:
+            return None
+
+        locale = locale or self.i18n_origin
+        cache_key = (pagename, locale)
+        page = self.CACHE.get(cache_key)
+
+        if page is not None:
+            return page
+
+        # not yet instantiated
+
+        fname = os.path.join(self.folder, locale, pagename) + '.md'
+        if not os.path.exists(fname):
+            logger.error('file %s does not exists', fname)
+            return None
+
+        page = self.page_class(fname, self.base_url)
+        self.CACHE[cache_key] = page
+        return page
+
+    def all_pages(self, locale: typing.Optional[str] = None):
+        """Iterate over all pages"""
+        locale = locale or self.i18n_origin
+        for pagename in self.toc:
+            page = self.get_page(pagename, locale)
+            yield pagename, page