[mod] user_help: support translations

replace "user_help.HELP" by a new function "get_help_for_locale".
This function the help a one locale, and fall back to English if the
translation is not available.

searx/user_help/__init__.py

@@ -0,0 +1,61 @@
+from typing import Dict
+import os.path
+import pkg_resources
+
+import flask
+from flask.helpers import url_for
+import mistletoe
+
+from .. import get_setting
+from ..version import GIT_URL
+from ..locales import LOCALE_NAMES
+
+HELP: Dict[str, str] = {}
+""" Maps a filename under help/ without the file extension to the rendered HTML. """
+
+
+def render(app: flask.Flask):
+    """
+    Renders the user documentation. Must be called after all Flask routes have been
+    registered, because the documentation might try to link to them with Flask's `url_for`.
+
+    We render the user documentation once on startup to improve performance.
+    """
+
+    link_targets = {
+        'brand.git_url': GIT_URL,
+        'brand.public_instances': get_setting('brand.public_instances'),
+        'brand.docs_url': get_setting('brand.docs_url'),
+    }
+
+    base_url = get_setting('server.base_url') or None
+    # we specify base_url so that url_for works for base_urls that have a non-root path
+
+    with app.test_request_context(base_url=base_url):
+        link_targets['url_for:index'] = url_for('index')
+        link_targets['url_for:preferences'] = url_for('preferences')
+        link_targets['url_for:stats'] = url_for('stats')
+
+    define_link_targets = ''.join(f'[{name}]: {url}\n' for name, url in link_targets.items())
+
+    for locale_name in pkg_resources.resource_listdir(__name__, '.'):
+        if locale_name not in LOCALE_NAMES:
+            continue
+        HELP[locale_name] = {}
+        for filename in pkg_resources.resource_listdir(__name__, locale_name):
+            rootname, ext = os.path.splitext(filename)
+
+            if ext != '.md':
+                continue
+
+            markdown = pkg_resources.resource_string(__name__, locale_name + '/' + filename).decode()
+            markdown = define_link_targets + markdown
+            HELP[locale_name][rootname] = mistletoe.markdown(markdown)
+
+
+def get_help_for_locale(locale_name: str) -> Dict[str, str]:
+    result = {**HELP['en']}
+    if locale_name != 'en':
+        for rootname, content in HELP.get(locale_name, {}).items():
+            result[rootname] = content
+    return result

searx/user_help/en/about.md

@@ -0,0 +1,71 @@
+# About [searxng][url_for:index]
+
+SearXNG is a fork from the well-known [searx] [metasearch engine], aggregating
+the results of other [search engines][url_for:preferences] while not storing
+information about its users.
+
+More about SearXNG ...
+
+* [SearXNG sources][brand.git_url]
+* [weblate]
+
+---
+
+## Why use it?
+
+* SearXNG may not offer you as personalised results as Google,
+  but it doesn't generate a profile about you.
+
+* SearXNG doesn't care about what you search for, never shares anything
+  with a third party, and it can't be used to compromise you.
+
+* SearXNG is free software, the code is 100% open and you can help
+  to make it better.  See more on [SearXNG sources][brand.git_url].
+
+If you do care about privacy, want to be a conscious user, or otherwise
+believe in digital freedom, make SearXNG your default search engine or run
+it on your own server
+
+## Technical details - How does it work?
+
+SearXNG is a [metasearch engine], inspired by the [seeks project].  It provides
+basic privacy by mixing your queries with searches on other platforms without
+storing search data. Queries are made using a POST request on every browser
+(except Chromium-based browsers*).  Therefore they show up in neither our logs,
+nor your url history. In the case of Chromium-based browser users there is an
+exception: searx uses the search bar to perform GET requests.  SearXNG can be
+added to your browser's search bar; moreover, it can be set as the default
+search engine.
+
+<span id='add to browser'></span>
+## How to set as the default search engine?
+
+SearXNG supports [OpenSearch].  For more information on changing your default
+search engine, see your browser's documentation:
+
+* [Firefox](https://support.mozilla.org/en-US/kb/add-or-remove-search-engine-firefox)
+* [Microsoft Edge](https://support.microsoft.com/en-us/help/4028574/microsoft-edge-change-the-default-search-engine)
+* Chromium-based browsers [only add websites that the user navigates to without a path.](https://www.chromium.org/tab-to-search)
+
+## Where to find anonymous usage statistics of this instance ?
+
+[Stats page][url_for:stats] contains some useful data about the engines used.
+
+## How can I make it my own?
+
+SearXNG appreciates your concern regarding logs, so take the code from
+the [SearXNG project][brand.git_url] and run it yourself!
+
+Add your instance to this [list of public instances][brand.public_instances] to
+help other people reclaim their privacy and make the Internet freer!  The more
+decentralized the Internet is, the more freedom we have!
+
+## Where are the docs & code of this instance?
+
+See the [SearXNG docs][brand.docs_url] and [SearXNG sources][brand.git_url]
+
+[searx]: https://github.com/searx/searx
+[metasearch engine]: https://en.wikipedia.org/wiki/Metasearch_engine
+[weblate]: https://weblate.bubu1.eu/projects/searxng/
+[seeks project]: https://beniz.github.io/seeks/
+[OpenSearch]: https://github.com/dewitt/opensearch/blob/master/opensearch-1-1-draft-6.md