[chrisjsewell]

Adds an optional syntax extension, section_ref, built on the mdit-py-plugins section_ref plugin (executablebooks/mdit-py-plugins#144).
It turns section-sign references such as §1, §1.1 and §2.3.4 into internal links to the correspondingly numbered heading of the current document:

# Title

See §1 and §1.1 for details.

## First Section

### Nested

This is blocked on mdit-py-plugins 0.7.0 reaching PyPI: pyproject.toml and the mypy pre-commit dependencies pin mdit-py-plugins~=0.7, so CI cannot install until then.
The test suite, pre-commit and a -nW docs build with the extension enabled all pass locally against mdit-py-plugins from the branch that adds the nine-digit limit per level
(the conformance fixture's digit cases need it, so 0.7.0 must include it).

How it works:

• The plugin emits a section_ref token whose meta["numbers"] holds the parsed number.
  The renderer turns it into an inline marker carrying those numbers.
• A ResolveSectionRefs transform numbers the document's headings as written:
  under a single top-level heading its subsections are §1, §2, ...; otherwise the top-level headings are; deeper levels add dots.
  It runs at priority 300, after the transforms that settle each section's first id (including PrioritiseExplicitIds, so (label)= ids win),
  and before docutils promotes a lone top-level heading to the document title (and a lone subsection to a subtitle), which Sphinx never does.
  So every reference resolves to the same heading in both, also with docutils' --section-subtitles.
• A section counts wherever the document tree places it, under its nearest section ancestor:
  inside an only node whatever its condition (so the numbers are the same for every builder), an ifconfig node, or any other container.
• A resolved marker becomes an internal reference to the section's primary id, with class section-ref, the heading's title as reftitle (a hover tooltip in Sphinx HTML),
  and the parsed number kept as section_numbers.
  An unresolved one stays as text marked with the section-ref class, with a suppressible myst.section_ref warning
  ("Section reference target not found in this document: '§9.9'", giving the number as written).
• A reference inside a section heading or link text is left unlinked, since a link there would nest inside another link (headings are copied into toctree and contents entries).
  References in other titles (admonitions, tables) and in rubrics (a heading inside a directive body) resolve as usual.
• An unresolved reference in an {include}d file is reported at that file's line.
• Image alt text keeps the reference as written (![a §1 b](x.png) gives alt="a §1 b").

The docs (docs/syntax/optional.md, "Section references") say that §-numbers are not the numbers a :numbered: toctree displays
(and point to {numref} with a label for citing those),
that legal citations (§242 BGB) and references into other specifications (RFC 9110 §8.3) are read as local references
(with suppress_warnings = ["myst.section_ref"] next to that caution),
that § writes a literal sign,
that numbers are positional (inserting a heading retargets later references without a warning, so use a (label)= target where that matters),
and that a reference in an included file resolves against the including document.
A "Numbering rules" subsection states the behaviour exactly: what text is a reference (including the nine-digit limit per level),
which headings count (included headings at their written level shifted by :heading-offset:, {eval-rst} sections, {only} headings whatever the condition, sections placed by other directives, a myst_title_to_header title),
how they are numbered, and how references resolve.

The rules are pinned by one conformance fixture, tests/test_renderers/fixtures/section_ref_resolution.md:
each case is a document plus the heading every reference resolves to (or that it is unresolved and warns, or inert and does not),
and one test runs every case through both docutils and Sphinx
(the cases that need {only}, {ifconfig} or a link to another document run in Sphinx only, since docutils has no such directive and cannot resolve such a link).
A reference into an {only} branch that the build excludes points to an anchor that is not in the output; the fixture deliberately does not pin it.
The fixture is plain data, so another implementation of the syntax can port it.

Resolution is document-local in this version. A follow-up could resolve against Sphinx's section numbers for :numbered: toctrees, or across documents.