docs: add code graph preview support reference

[oiahoon]

What

• add an evidence-backed support matrix for the v0.7.0 code graph preview
• distinguish fixture-backed, partially covered, and unsupported language behavior
• document Express resolution, graph-unavailable degradation, and current limitations
• link the reference from the README and extractor contributor guide

Why

Preview users and contributors need one tested reference that separates current behavior from planned language and framework work.

Closes #92

Type of change

• Bug fix
• New feature
• Refactor
• Docs
• CI/Tooling

How to test

1. Run npm run typecheck.
2. Run npm test.
3. Run npm run build.
4. Verify local Markdown links in the three changed files resolve.

Checklist

• Tests pass (npm test)
• No breaking changes (or documented below)
• Tested locally with a real project (not applicable: documentation-only)

Code-graph preview

• This PR targets code-graph-preview, not main
• The linked issue states Target branch: code-graph-preview
• The change follows the frozen LanguageExtractor or FrameworkResolver interface
• A focused fixture and assertions for the expected node/edge shape are included (not applicable: documentation-only; existing fixtures are referenced)
• Any new grammar WASM, extension mapping, extractor, or resolver is registered (not applicable: no new implementation)
• No graph identity, reconciliation, schema, or drift-semantics changes are included, or a core / discuss-first issue is linked above

[theDakshJaitly]

Verified the support matrix against the current grammar/extractor registries, focused fixtures, Express resolver, graceful-degradation paths, and ignored-source globs. The branch is updated with code-graph-preview, the multilingual README selector is preserved, and all new relative documentation links resolve.

[theDakshJaitly]

Thank you for the contribution @oiahoon
we recently made a discord server for the repo, feel free to join: discord link