[DISCUSS] Centralising version-independent component documentation

Hi Everyone,

Component documentation is currently only published to
https://maven.apache.org/ when a component releases. This makes
documentation fixes for rarely released components, or updates to already
released components, effectively complicated.

I would like to propose moving the source of version-independent narrative
documentation into maven-site.git, where a commit would republish the
changes immediately.

To understand the scope, I measured the current estate: there are 1,068
hand-written site pages across our repositories. While 521 are Velocity
pages, 396 of those use nothing beyond component identity, version, and
escapes. This means 88% of the pages are portable if maven-site can supply
a component's current version to a page. We already half-do this today:
Update.java maintains the version columns of the plugins and shared index
pages, though it currently gets its numbers by scraping the component's own
published page. This would need to be re-based on an authoritative source
first. The site checkers in maven-dist-tool share this same dependency.

Importantly, published URLs would not break under this approach:

  - Where a component's generated output is a small, stable set of
directories, maven-site can own the path and symlink only those back,
leaving the URL untouched.
  - Where this is not possible, a redirect in our existing .htaccess file
can be used. For example, the maven-checkstyle-plugin redirects have
successfully survived many releases.
  - Archived, versioned sites would remain unaffected either way.

I have written up the full details, measurements, and open questions here:
https://github.com/apache/maven-site/issues/1647

I am particularly interested in refreshing this discussion now, as I have
recently moved many pages from APT to Markdown, which should make future
contributions and refactoring much easier.

Regards,
Sylwester