Allow safe connector MDX formatting

[gontzess]

Why
Connector docs validation was flagging valid docs syntax, including comment-only MDX expressions, table <br/> line breaks, CardGroup, and indented code placeholders. This keeps the PR check aligned with docs syntax we expect while preserving rejects for executable MDX and unsafe URLs.

What this changes

• Allows comment-only MDX expressions, safe <br/> elements, and CardGroup.
• Adds a shared fixture file for cases that should match registry validation.
• Adds reject fixtures for reviewed escape/comment bypass cases so the workflow validator pins the same contract as the registry validator.
• Runs the fixture contract from the Node linter tests.

Validation:

• npm test in tools/mdx-lint: 30 passed.
• Live connector-doc compatibility scan: 233 valid, 1 real doc issue, 71 missing docs.

Rollout:

• After merge, connector repos need to consume this workflow ref/tag before PR checks see the updated policy.