Fix two broken links and the wrong mailing list

[slachiewicz]

Part of codehaus-plexus/.github#58. The smallest, least arguable piece of the site cleanup — three corrections, seven lines deleted, nothing added.

writing-components-trail/index.html → 404

The Books → Writing Components entry in site.xml points at a path that doesn't exist. Because it's in the site descriptor, it renders in the left menu of every page on the site, so this is the most-hit broken link we have.

Its only child duplicated Writing Plexus Components (/guides/writing-components/00_index.html), which is directly below it and works. So the Books group is removed rather than repointed — nothing becomes unreachable.

faq.html → 404

index.apt closes with "For answers to common questions about Plexus, see the FAQ." There is no FAQ page and never has been in this repo. Removed the sentence; writing an actual FAQ is a separate question, not a link fix.

Mailing list → wrong organisation

Community → Mailing Lists points at groups.google.com/g/mojohaus-dev. That's MojoHaus — a different org. Anyone following it to ask a Plexus question lands in the wrong place.

Plexus has no mailing list of its own, so rather than invent one the entry now points at the GitHub org, where issues actually get handled. (This matches the "where to ask" section of the CONTRIBUTING in codehaus-plexus/.github#60.)

Verification

Built the site and walked every internal link in the output. Both targets are gone and nothing new broke.

That scan also turned up 6 pre-existing broken links inside the archived IoC guides, which I've deliberately left alone here:

guides/developer-guide/configuration/component-descriptor.html -> ./component-descriptor-specification.xml
guides/quick-start/one-minute-tutorial.html                    -> ./plexus-lifecycle.html
guides/trails/index.html                                       -> ./plexus-web-applications/index.html
guides/trails/index.html                                       -> ./writing-components/index.html
ref/why-use-components.html                                    -> ./components.html
ref/available-components.html                                  -> /plexus-cli

The last one goes away with the component-index rewrite. The rest are inside historical content and can be a follow-up if anyone thinks they're worth fixing.