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

[jdsika]

Summary

The SHACL generator translates LinkML rules: into sh:sparql constraints, but
the generator documentation did not mention it — so the feature was
undiscoverable and its limits undocumented.

Adds a Rule constraints (SHACL-SPARQL) section to docs/generators/shacl.rst
covering:

• the recognised named patterns (boolean guard, presence implies value,
  exclusive value) and the compositional fallback;
• the --emit-rules / --no-emit-rules flag (default: on);
• the skip, never mis-translate contract, and which rule attributes warn
  (bidirectional, open_world, elseconditions) versus are silently skipped
  (deactivated);
• a worked YAML → Turtle example;
• the note that sh:sparql needs a processor with SHACL-SPARQL support
  (e.g. pyshacl with advanced=True).

Documentation only — no source or test changes.

Stack position

main
└─ #19  presence-implies-value
   └─ #20  compositional fallback (M1–M5)
      └─ #23  documentation   ← this PR

Important

Depends on #19 and #20. This section documents behaviour introduced by
both, so it should merge last. If #20 is deferred, the paragraph describing the
compositional fallback needs to be dropped from this PR.

[jdsika]

Mirrored upstream as linkml#3991.