Point contributors at CONTRIBUTING; label the retired container docs

[slachiewicz]

Part of codehaus-plexus/.github#58. Last of the three site PRs.

Stacked on #18, which is stacked on #17. Both retarget automatically as the earlier ones merge. Review order: #17 → #18 → this.

Get Involved was actively misleading

The page currently tells a would-be contributor to:

• discuss on the Plexus mailing lists — there are none, and haven't been for years
• paste a Copyright 2007 The Codehaus Foundation header into new files
• download maven-idea-codestyle.xml into ~/.IntelliJIDEA/config/codestyles — a path from IntelliJ 8, for a formatter config the build stopped using when it adopted Spotless

That last one has a real cost: formatting is enforced by spotless:check with palantir-java-format, and nothing anywhere tells a first-time contributor to run mvn spotless:apply. So their first PR goes red on formatting, and the one page that claims to explain code style sends them somewhere else entirely.

The page now carries a short version — where to ask, mvn verify, mvn spotless:apply, licence headers, PR expectations — and links to CONTRIBUTING.md (codehaus-plexus/.github#60) for the rest. One contributor document instead of two that disagree.

It also now says explicitly that there is no mailing list, because older pages on this site point at one and people do follow those links.

The codestyle files stay served. They're simply no longer advertised. Anyone with an existing link still gets the file.

Menu

Old IoC Documentation → Plexus Container (retired). Says what the section is, not just that it's old, and names the thing that was retired so the label is useful to someone who arrived from a fifteen-year-old blog post.

Added Contributing and Security to the Community menu.

What this PR deliberately does not do

No page is moved, renamed or deleted, and no URL changes. I've put a comment in site.xml saying so, next to the retired section, so the next person to tidy this doesn't have to re-derive it:

<!-- Historical documentation for the retired Plexus IoC container, replaced by Eclipse Sisu.
     Kept, and kept at these exact URLs, because a great deal of external writing links to it.
     Do not move or delete these pages. -->

I considered adding a visible "this is historical" banner to the ~45 pages in that section. It needs editing every page for a message the menu label now carries, so I've left it. Easy to add later if reviewers think the label isn't enough.

Verification

Site builds; link check unchanged at the same 5 pre-existing broken links inside the historical guides. Confirmed codestyle/maven-eclipse-codestyle.xml and codestyle/maven-idea-codestyle.xml are still present in the built output.