www: updates and fixes, added security doc references.

[cederom]

Summary

• Updated top menu to fix invalid render on non mobile vertical screens. Top menu now contains: Community, Read, Code, Apache top level items. This allows proper website rendering and adding information to sections.
• Added reference to Security in Documentation (index, read/security).
• Updated articles page (formatting, chronologic sort, pdf+url references). This improves display and adds external references to published scientific papers that should help authors in citations / references / bibliography.
• Renamed article pdf files to contain date author and title in filename, this will help in file search and referencing.
• Changed articles pdf location to /static/articles/ (req for static files).
• Fixed pages coherence (the same section levels, removed trail blank lines) in the community, community-members, articles pages.
• Note: Documentation links still point to baseurl so that local documentation builds can be tested locally (localhost:4000) when testing offline.

Impact

• Provides reference to security documentation.
• Fixes rendering and layout issues.
• Adds scientific papers and articles online references for citation.
• Renames and moves articles pdf files to proper /static/articles URL dedicated for static files.

Testing

% uname -a
FreeBSD hexagon 14.3-RELEASE-p7 FreeBSD 14.3-RELEASE-p7 GENERIC amd64

% bundle exec jekyll serve
Calling `DidYouMean::SPELL_CHECKERS.merge!(error_name => spell_checker)' has been deprecated. Please call `DidYouMean.correct_error(error_name, spell_checker)' instead.
Calling `DidYouMean::SPELL_CHECKERS.merge!(error_name => spell_checker)' has been deprecated. Please call `DidYouMean.correct_error(error_name, spell_checker)' instead.
Configuration file: /XXX/nuttx-website.git/_config.yml
            Source: /XXX/nuttx-website.git
       Destination: /XXX/nuttx-website.git/target
 Incremental build: disabled. Enable with --incremental
      Generating...
     Build Warning: Layout 'news_item' requested in _posts/2019-12-19-hello.md does not exist.
Deprecation Warning [import]: Sass @import rules are deprecated and will be removed in Dart Sass 3.0.0.
(..)
    Server address: http://127.0.0.1:4000
  Server running... press ctrl-c to stop.

• Render before:
  
• Render after: