Moved documentation from wiki to GitHub repo + modern Jenkins core

[rsandell]

Following the instructions and motivations here https://jenkins.io/blog/2019/10/21/plugin-docs-on-github/

Also updated core to slightly more modern incarnation to avoid strangeness with new plugin pom version.

[alecharp]

Those elements are not rendered by Github Markdown, and won't appear on the plugin page.
But it's not breaking the page either.

[rsandell]

Strange, the conversion guide explicitly mentioned it.

[alecharp]

then I might be wrong, but the "rich diff" in the PR files view do not display the TOC. Also, it's adoc, not md so you must be correct.

[MarkEWaite]

GitHub renders the table of contents (for example, Git plugin) as does the plugins.jenkins.io site. Unfortunately, I have not found a way to have both badges at the top and a table of contents with adoc. If I insert the badges at the top, then the table of contents is not rendered. If I insert the table of contents followed by the badges, the badges are below the table of contents and much less useful.

I'm now considering if I should switch to a format more like the credentials plugin with a top-level docs README that is short and includes badges, with links to the larger topics (a self-maintained table of contents). I really like the badges and would prefer that every plugin has them visible so that users become accustomed to reading and using them.