2.x Package Documentation Support

[mbabker]

Infrastructure and UI changes needed for supporting rendering package docs on this website.

• Adds symfony/cache package to cache rendered markup
• Uses joomla/github to render markup via GitHub API in appropriate context
• Uses GitHub's syntax highlighting CSS for code examples
• Adds UI for a page type with a primary section and a sidebar, flex/grid should be done in a way where the sidebar is rendered after the main body on mobile devices
• Adds info to database to determine if a package exists on a major version
• Docs section routing:
  • /docs/:version/:package/:filename is the general pattern, where:
    • :version is either a major version branch (1.x or 2.x) or the shortcut "latest" (which will redirect to 2.x for now)
    • :package is the package name
    • :filename is the path to the file within a repo's docs directory (when omitted it redirects to the overview page)

[Hackwar]

I've pushed the latest changes from master into this branch to bring it up to date. @nibra @HLeithner maybe you want to have a look at this and if we want to adopt this finally?

[Hackwar]

The question is: Who is going to finish this PR and who is going to write all that documentation?