Groups hexdocs pages by kind and adds a manifest

[johnnyt]

What changes

The hexdocs sidebar groups each shipped page by the kind of page it is,
in the family order, where one "Guides" group held them all:

• How-to guides: the pages under docs/guides/.
• Explanation: docs/explanation/moving-off-cloak.md.

The README (still main) and the CHANGELOG stay ungrouped at the top.
The extras list, the package files and the API reference do not change;
no decision record, plan or other contributor path was ever an extra
here, and none is added.

Two how-to pages take "How to" titles; their files keep their names and
docs/guides/ keeps its name:

• two-vaults-customer-and-agreement.md: "How to keep a customer scope
  and an agreement scope in two vaults".
• scope-in-jobs-and-projectors.md: "How to resolve the scope in jobs
  and projectors".

The link text for those two pages in README.md and docs/README.md
follows the new titles; every link target is unchanged. The two guide
tests read only the pages' code blocks, so the titles do not reach them.

.claude/diataxis.md is new: the docs manifest the documentation tools
read (audience, where each kind of page lives, the contributor paths,
the executed snippets). Examples in this package are domain-free. It is
the generated file, byte for byte; neither prose line was changed.

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

Checks

• Full mix quality green on the committed tree, including the Docs
  stage (ExDoc warnings as errors) and the doc_links stage.
• The built sidebar was read back: README, Changelog and API Reference
  ungrouped; every docs/guides/ page under "How-to guides"; the
  explanation page under "Explanation".

Review

Gate tier, the worker's own review round: docs configuration under the
size threshold, no contract surface.