[doc] improve documentation of make targets and ./manage script

BTW force modularization of the ./mange script into sub modules:

- utils/lib_sxng_data.sh
- utils/lib_sxng_node.sh
- utils/lib_sxng_static.sh
- utils/lib_sxng_test.sh
- utils/lib_sxng_themes.sh

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

utils/lib_sxng_node.sh

@@ -0,0 +1,51 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+export NODE_MINIMUM_VERSION="16.13.0"
+
+node.help(){
+    cat <<EOF
+node.:
+  env       : download & install SearXNG's npm dependencies locally
+  env.dev   : download & install developer and CI tools
+  clean     : drop locally npm installations
+EOF
+}
+
+nodejs.ensure() {
+    if ! nvm.min_node "${NODE_MINIMUM_VERSION}"; then
+        info_msg "install Node.js by NVM"
+        nvm.nodejs
+    fi
+}
+
+node.env() {
+    nodejs.ensure
+    (   set -e
+        build_msg INSTALL "./searx/static/themes/simple/package.json"
+        npm --prefix searx/static/themes/simple install
+    )
+    dump_return $?
+}
+
+node.env.dev() {
+    nodejs.ensure
+    build_msg INSTALL "./package.json: developer and CI tools"
+    npm install
+}
+
+node.clean() {
+    if ! required_commands npm 2>/dev/null; then
+        build_msg CLEAN "npm is not installed / ignore npm dependencies"
+        return 0
+    fi
+    build_msg CLEAN "themes -- locally installed npm dependencies"
+    (   set -e
+        npm --prefix searx/static/themes/simple run clean
+    )
+    build_msg CLEAN "locally installed developer and CI tools"
+    (   set -e
+        npm --prefix . run clean
+    )
+    dump_return $?
+}