docs: Update the architecture diagrams by nickvigilante · Pull Request #25816 · coder/coder

Several relative links in the docs pointed at pages that no longer exist
or rendered incorrectly on coder.com.

Fixes:

- `start/first-template.md`: IDE links repointed from the removed
`../ides.md` / `../ides/web-ides.md` to their current homes under
`user-guides/workspace-access/`.
- `tutorials/example-guide.md`: contributing link repointed to
`../about/contributing/documentation.md`.
- `about/contributing/backend.md`: the `migrations/testdata/fixtures`
and `full_dumps` references (and the `000024_example.up.sql` example)
used relative paths that escape `docs/` and render as bogus
`/docs/coderd/...` routes on the site. Normalized to the canonical
`github.com/coder/coder/(blob|tree)/main/...` form already used by ~120
other source links in the docs.
- Normalized extensionless directory links (`ai-coder/ai-gateway`,
`user-guides/workspace-access`, `install`) to their `/index.md` targets
for consistency with the rest of the docs.

This class of bug is invisible to the local doc checks (`make
lint/markdown` / `pnpm check-docs` only run markdownlint + table
formatting); only CI's Linkspector job validates link targets. Found via
a relative-link audit while investigating the docs preview on #25816.

Source-link version-awareness (so older docs versions don't all point at
`main`) is tracked separately in DOCS-268 and will be handled in the
coder.com render layer.


Linear: DOCS-278

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>