[typespec-ts] Add reference documentation for diagnostics

## Summary

Add self-service reference documentation for diagnostics emitted by `@azure-tools/typespec-ts`, following the pattern introduced for TCGC in #4982.

`packages/typespec-ts/src/lib.ts` currently defines 42 diagnostics, but they do not have source documentation or published diagnostic URLs.

## Proposed work

- Add one source Markdown file per reachable diagnostic under `packages/typespec-ts/src/diagnostics/`.
- Document the diagnostic's impact, triggering usage, message, and how to fix it.
- Document suppression guidance for warnings where applicable.
- Add a helper in `src/lib.ts` that sets each diagnostic's `docs` source and published `url`.
- Update `.gitignore` to ignore generated TypeSpec TS diagnostic reference pages under `website/src/content/docs/docs/emitters/clients/typespec-ts/reference/diagnostics/`.
- Run `pnpm --filter @azure-tools/typespec-ts regen-docs` and confirm that every diagnostic produces a page without missing-documentation warnings.
- Audit diagnostics without report sites and internal invariant diagnostics. Any removals, `compilerAssert` conversions, or severity changes should be justified independently rather than copied automatically from TCGC.

## Reference

- #4982

Start by comparing the TCGC documentation pattern from #4982, then inspect packages/typespec-ts/src/lib.ts and the diagnostics under packages/typespec-ts/src/diagnostics/. Run pnpm --filter @azure-tools/typespec-ts regen-docs and verify every reachable diagnostic produces a page without missing-documentation warnings. Done includes the .gitignore update and an independently justified audit of diagnostics without report sites and internal invariant diagnostics.

Written by the indexing model from the issue text.