[FEATURE] Document the :changelog: option of the version directives

[CybotTM]

Documents the :changelog: option that render-guides#1303 added to versionadded, versionchanged and deprecated.

Draft on purpose. The option is in render-guides main but not in 0.41.0, which is what renders docs.typo3.org today — I checked the tag rather than assuming. This should not merge before a release that carries it. Everything else is ready.

The page currently shows only the hand-written permalink in the directive body, which is exactly what the option replaces, so the new section sits after the existing ones and says so. The link text the option produces is the entry's own title, i.e. what an author writes by hand today; the difference is that a non-existent entry now produces a build warning and the unresolved-reference marker instead of a permalink that leads nowhere.

Covered: the three value forms (Core entry id, another manual's interlink shortcode plus anchor, the local #anchor), and the embedded text <entry> form for the case where the resolved title does not describe the change — an extension whose whole changelog page carries one label, for instance.

Every example is a code-block, none of them a live directive, so the page renders unchanged on the current release and nothing has to be revisited when the option ships. make test-docs passes with no warnings.

Assisted by claude-code:claude-opus-5 — Session

[CybotTM]

Out of draft — the content is complete and documentation is green.

One thing for whoever merges: this should not land before a render-guides release that carries the option. It is in main (render-guides#1303, merged as 3a35e418) but not in 0.41.0, which is what renders docs.typo3.org today — I read the tag rather than assuming. Nothing breaks if it merges early: every example is a code-block, not a live directive, so the page renders unchanged. It would just describe an option authors cannot use yet.

A re-read against the repo's own conventions turned up three things, all fixed in f49c4b9: the heading underline was one character short of the title, the section used -- where this file and the manual around it use - (84 occurrences against 8), and "the option takes three forms" read as if the embedded text <entry> form were a fourth — the three are ways to address the entry, and the embedded form wraps any of them.

make test-docs passes with no warnings.