docs(shacl): document rule-to-SHACL-SPARQL constraint generation

[jdsika]

Mirror of linkml#3991, same head. Review there; this PR tracks the fork stack.

Summary

This PR documents the translation of LinkML rules into SHACL-SPARQL in docs/generators/shacl.rst, which did not mention it. It also replaces the dangling "See above for implementation status" in the rules section of docs/schemas/advanced.md with links to the JSON Schema and SHACL generator pages.

The new section covers:

• the two named patterns, and the composed translation with its supported operators (feat(gen-shacl): translate presence-implies-value rules to SHACL-SPARQL linkml/linkml#3989, feat(gen-shacl): add a compositional fallback for rule-to-SPARQL conversion linkml/linkml#3990);
• the semantics shared with jsonschemagen:
  • presence;
  • "every value" versus has_member;
  • value comparison (enum meaning, the boolean lexical forms);
  • numeric bounds;
• inheritance into subclass shapes, open_world, --no-emit-rules, and the rule that a rule is skipped with a warning rather than partially translated;
• an example generated by gen-shacl from this stack, and what its results report (sh:resultPath, sh:value).

Corrections to the earlier version

• Skipped rules are logged as warnings, not at DEBUG.
• open_world is honoured.
• Values are compared with =, not NOT IN.
• pyshacl validates sh:sparql constraints without advanced=True.

Verification

• The example's output comes from gen-shacl, and its documented results were checked with pyshacl.
• make html passes with SPHINXOPTS="-W --keep-going", as in docs-test.

Documentation only.

Upstream stack: linkml#3989 → linkml#3990 → linkml#3991 (docs).
Fork stack: #19 → #20 → #23 (docs); #35 (tests) is also based on #20.

[jdsika]

Mirrored upstream as linkml#3991.