fix(docs): restore table rendering in MDX pages

[Christopher MANEU (cmaneu)]

Summary

Fixes #1427.

• Explicitly enable GitHub Flavored Markdown so the installed MDX integration renders the profile anatomy as a table instead of pipe-delimited text. The integration does not inherit Astro's Markdown processor defaults.
• Add built-output regression tests covering all profile fields, headers, cells, links, and bold text, plus existing plain Markdown tables.
• Run these checks automatically from pnpm build, using the existing CI workflow without workflow or dependency changes.

Demos

N/A — documentation-site rendering only; no Portal or CLI changes. Verified the generated HTML directly.

Before

The profile guide contained zero HTML tables; the new regression test failed with the reported pipe-delimited output.

After

The guide contains a three-column table with all eight fields. The profile schema's Markdown tables and HTTP example tabs remain intact.

Testing

From website/:

• pnpm test — all 7 plugin tests passed.
• SITE=https://microsoft.github.io BASE_PATH=/scope pnpm build — all 203 pages built, then both rendered-table regression tests passed automatically.
• Confirmed the MDX regression test fails on the original build and passes after the fix.
• git diff --check — passed.

Documentation and compatibility

Updated website/README.md with the explicit GFM requirement and the build-output test command. No API, CLI, schema, dependency, or deployment URL changes. The explicit GFM setting preserves compatibility with the currently installed MDX integration; a broader dependency upgrade is intentionally out of scope.

Checklist

• N/A — Portal/CLI feature parity; no product features changed.
• N/A — Storybook; no Portal components changed.
• N/A — database migration; no database changes.
• N/A — lockfile/notices; no dependencies changed.
• N/A — before/after video; documentation rendering is covered by generated-HTML regression tests.