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

[jdsika]

Summary

Adds the narrative documentation for the rules → SHACL-SPARQL constraint
generation to docs/generators/shacl.rst: the --emit-rules switch, the
three named patterns, the compositional fallback, the skip, never
mis-translate contract, a schema → Turtle example, and the pyshacl
advanced=True requirement. The CLI options themselves are already covered
by the page's .. click:: directive; this section documents the behaviour.

Closes the documentation gap flagged in the PR-guideline audit (new generator
features must be documented in docs/generators/). Documentation-only —
no generator code changes.

Stack position

main
└─ feat/shaclgen-rules-sparql                      #11 (green)
   └─ feat/shaclgen-presence-implies-value-stacked   #19
      └─ feat/shaclgen-compositional-rule-fallback   #20
         └─ fix/shaclgen-rule-converter-hardening    #21
            └─ fix/shaclgen-rule-converter-audit-findings #22
               └─ docs/shaclgen-rule-constraints    ← this PR

When upstreaming, carve the relevant paragraphs into the submission that
introduces each behaviour (named patterns → framework/PIV; fallback → M1–M5).

How was this tested?

• Documentation-only change; the RST section follows the page's existing
  heading levels and code-block conventions.
• The example output matches what the generator at this branch's tip emits
  for the example schema (verified against the test-suite fixtures).

Areas of uncertainty

• Placement: the section sits under Docs before Command Line; maintainers
  may prefer a separate page as the pattern set grows.

Checklist

• My code follows the contributor guidelines
• I have added tests that prove my fix/feature works (n/a — docs only)
• Existing tests pass locally with my changes

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