[enh] engines: add about variable

[dalf]

What does this PR do?

move meta information from comment to the about variable
so the preferences, the documentation can show these information

the different URL have been checked and updated (especially API documentation URL).

Also:

• the UTF8 encoding header is removed. According to https://docs.python.org/3/howto/unicode.html Python supports writing source code in UTF-8 by default.
• add the SPDX License Identifier

Why is this change important?

See #2442

Currently the engine list in /prefereneces or in the documentation show a list of name without a description or a link.

This PR adds the required information to implement this feature, but I suggest to implement the UI in another PR.

Without this PR, the top head comment in an engine provides some information:

• @website : the user URL, exactly what would be nice to display to the user.
• @provide-api : does the actual engine provide an official API ? usually the source code include an URL.
• @using-api: does searx use this API ?
• @results: what kind of data searx deal with ? HTML, JSON, XML
• @stable: is the searx engine stable ?
• @parse: what kind of data the searx engine return ?

IMO:

• @stable without update will be wrong:
  • an official API can change.
  • the HTML layout of an engine can remain stable over time.
  • I suggest to remove this attribute : @results = HTML is enough to understand that searx engine is "unstable".
• @parse: is it useful ? I suggest to remove this attribute

In this PR, these attributes are replaced by a variable:

about = {
    "website": 'https://arxiv.org',   # official user URL
    "wikidata_id": 'Q118398',   # wikidata identifier : see https://www.wikidata.org/wiki/Q28134166
    "official_api_documentation": 'https://arxiv.org/help/api',  # API documentation URL 
    "use_official_api": True,  # does the engine use the official API ?
    "require_api_key": False,  # does the engine requires a key ?
    "results": 'XML-RSS',  # same as before : what kind of data the searx engine deal with ?
}

The wikidata_id is interesting for different purposes:

• check the user URL
• automatically get a description of the engine in different languages (and logo?), see https://gist.github.com/dalf/db85f103de9c28b5fcb7896525a6d41b

How to test this PR locally?

• For now, the about is not used, there is nothing to test.

Author's checklist

Related issues

• Related to [UX] Additional info on search engines in preferences/engines #2442
• Related to Display which engines don't use HTTPS #302 (comment)
• Related to SPDX License Identifier #1801

WIP

• settings.yml : add about section for engines using the json or xpath engines.
• check the about validity in searx.engines.__init__

[unixfox]

LGTM and great changes!