Groups the hexdocs pages by kind of page

[johnnyt]

What changes

The hexdocs sidebar groups its extra pages by the kind of page a reader needs, in the family order, using the groups this package has pages for: How-to guides, Reference, Explanation. The README stays the main page; the README and the CHANGELOG stay ungrouped at the top. The API reference (the module pages) is unchanged.

• How-to guides: custom functions, embedding compiled programs, porting. Each H1 now starts "How to"; no file is renamed and docs/guides/ keeps its name.
• Reference: the language reference, the AST reference, the ISA reference, and three pages under docs/guides/ that describe a surface rather than a task: nested data access, location expressions, the simple subset.
• Explanation: the architecture page.

docs/contributing.md and LICENSE leave the extras (neither has a group in the family order) and still ship in the package files. The README's links to both become absolute GitHub URLs, because the doc links stage fails a relative link from an extra to a page that is not an extra. Decision records and planning notes were already outside the extras and the package files; that does not change.

Adds .claude/diataxis.md, the docs manifest the documentation tools read, generated for this repository and unedited. Its example world is the library loan.

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

Provenance

• docs/contributing.md and LICENSE leaving the extras, with the README linking them absolutely: decided by the conductor under a standing consent, 2026-10-05.
• Grouping nested data access, location expressions and the simple subset as Reference rather than How-to guides: an engineering choice made in this PR. Each reads as a description of an API surface (notations, valid targets and error types, a value and its laws), not as steps toward a goal, so a "How to" title would not fit them.

Checks

• Full mix quality green on this head, including the Docs stage (warnings as errors) and the Doc links stage.
• test/docs_adr_links_test.exs reads the extras from mix.exs; its floors still hold, because the two pages that left carry no ADR link and no bare docs/ citation.
• A local mix docs --warnings-as-errors build shows the README, the CHANGELOG and the API reference ungrouped, and every other extra in exactly one of the three groups.
• The two new absolute README links answer 200.

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