Groups hexdocs extras by kind of page

[johnnyt]

What changes

• mix.exs gains groups_for_extras. The one guide hexdocs ships, docs/describing-a-document.md, sits under How-to guides. The README (still main) and the CHANGELOG stay ungrouped at the top. extras and groups_for_modules are unchanged, so the API reference reads as before.
• No page under docs/adr, docs/plans or docs/spikes is in extras today, and none is added; package.files is unchanged and names nothing under docs/. The README links the decision records by absolute GitHub URL.
• How-to pages take H1s that start "How to". docs/describing-a-document.md already did; docs/guides/flow-patterns.md, docs/profiles.md and docs/theming.md change their H1 line only. No file is renamed and docs/guides/ keeps its name.
• .claude/diataxis.md is new: the docs manifest the documentation tools read, generated from the family's manifest table and committed byte for byte as generated. Its example world is patron registration (a visitor becomes a library patron through a few screens).

No file under lib/ changes. No changelog fragment: changelog.d/README.md excludes documentation changes.

Provenance

• The three H1 changes on pages hexdocs does not ship are this PR's reading of the acceptance's "a how-to page's H1 starts with How to", which does not limit itself to shipped pages; each is a one-line title change and no test reads those titles.
• Leaving the pages that do not ship today out of extras was decided under the night rule by the conductor, 2026-10-05; the rewritten acceptance on docs/adr was ruled by the operator, 2026-10-05.

Checks

• mix quality, full profile, green: the Docs stage (mix docs, warnings as errors) and the Doc links stage included.
• A local mix docs build's sidebar puts the describing-a-document guide under How-to guides and leaves the README and the CHANGELOG ungrouped; the module groups are as on main.
• The README is unchanged, and the files its links name are neither moved nor renamed.