Rewrite the overview as the component index, retire available-components

[slachiewicz]

Part of codehaus-plexus/.github#58.

Stacked on #17 — based on docs/fix-broken-links because both touch index.apt. GitHub will retarget this to source automatically once #17 merges. Review #17 first; it's seven deleted lines.

We currently document our own component list in two places, and both are wrong in different directions.

The overview page

Omits three maintained projects outright — plexus-sec-dispatcher, plexus-testing, plexus-build-api — while still listing the archived plexus-digest, whose site was last published in September 2015. Three entries (plexus-digest, plexus-i18n, plexus-interactivity) have nothing after the colon at all.

Now: every active project, grouped as Libraries / Components / Tooling, each with one line saying what it actually does, plus a Retired section that names a replacement for each archived repo rather than just dropping them. plexus-cipher → plexus-sec-dispatcher 4.x, which absorbed it (Cipher and internal/cipher live there now).

The "container is retired, Sisu replaced it" point moves up to the top. It was already on the page — but below a horizontal rule, after the component list, under a heading reading IMPORTANT NOTICE. It's the first thing someone needs to know, and it's also the thing the rest of our documentation keeps contradicting: plexus-i18n and plexus-interactivity currently render "The Plexus project provides a full software stack for creating and executing software projects" on their own homepages, two clicks from here.

ref/available-components.html

Worse. It advertises seven components that no longer exist in any form:

plexus-cli · plexus-spring · plexus-contextualizer · plexus-xwork-integration · plexus-mail-sender · plexus-cache · plexus-swizzle

…and lists none of the ones we actually maintain. It's linked from the site menu on every page, so it isn't obscure.

Content replaced with a short pointer to the overview, so there is one index instead of two.

The page is kept, not deleted. Its URL has been live for many years and is linked from outside; removing it would turn other people's working links into 404s. Same principle applies to the whole Old IoC Documentation section — nothing there moves in this PR or any later one.

Verification

Site builds. Walked every internal link in the output: the dead /plexus-cli reference is gone, and the five remaining broken links are pre-existing ones inside the historical guides (listed in #17). Confirmed all six newly-linked project sites return 200, including the three that were previously missing from the page.

Please check

The one-line descriptions — I wrote them against the source, since several projects have no usable description anywhere to copy. If the person who knows a component best reads its line and winces, please just say so.