docs: data-driven system-catalog reference, phase 4 (mz_introspection raw variants)

[antiguru]

Motivation

Phase 4 of the system-catalog docs data migration.
Stacked on phase 3 (base branch catalog-docs-data-phase3).

Description

Acknowledges the 19 mz_introspection _raw relations (internal differential-dataflow logging inputs) as raw variants on their parent's YAML entry, removing their RELATION_SPEC_UNDOCUMENTED markers.
A raw variant is name-plus-kind only, with no column table; the shortcode groups a relation's raw siblings into one "Raw relations" warning section (for example the nine raw inputs under mz_arrangement_sizes).
Lint treats raw variants as existence-only, so this is a pure move: the generated test/sqllogictest/autogenerated/mz_introspection.slt is unchanged.
Each raw relation's parent is the documented section it sits under in the markdown, resolved with a generator that resets attribution at the reference-link-definition block so the trailing orphan cluster is not misattributed.

mz_dataflow_operator_reachability (and its per-worker/raw siblings) and two orphan per-worker relations whose parents live in mz_internal stay as RELATION_SPEC_UNDOCUMENTED, pending phase 5 and subject-matter input.

Verification

ci/test/lint-docs-catalog.sh reports zero diff (pure-move proof), bin/sqllogictest passes, and the two-build hugo + htmltest gate is clean (895 documents, 0 errors). The generator has unit tests (ci/test/test_gen_raw_variants.py).