docs: fix broken references and add users oidc-claims to manifest (#25706)

## Summary

Three small docs fixes:

- **`docs/admin/integrations/oauth2-provider.md`**: Replace broken
relative link to `scripts/oauth2/README.md` with an absolute GitHub URL.
The previous link escaped the `docs/` tree
(`../../../scripts/oauth2/README.md`) and does not resolve in the
published docs site.
- **`docs/install/releases/feature-stages.md`**: Point the "Coder
documentation" link to `docs/about/contributing/documentation.md`. The
previous `../../README.md` target does not exist under `docs/`.
- **`docs/manifest.json`**: Add the missing `users oidc-claims` entry
alongside the other `users` CLI subcommands so the generated reference
page (`docs/reference/cli/users_oidc-claims.md`) is reachable from the
sidebar.

## Validation

- Confirmed each new link target exists on `main`
(`docs/about/contributing/documentation.md`, `scripts/oauth2/README.md`,
`docs/reference/cli/users_oidc-claims.md`).
- Pre-commit hooks pass (`fmt/markdown`, `lint/markdown`, `lint/emdash`,
`lint/typos`, etc.).

---

_This PR was prepared by a [Coder Agents](https://coder.com/) session on
behalf of @nickvigilante. Human review requested since this is a
docs-only change._

docs/admin/integrations/oauth2-provider.md

@@ -239,7 +239,7 @@ eval $(./setup-test-app.sh)
 ./cleanup-test-app.sh
 ```
 
-For more details on testing, see the [OAuth2 test scripts README](../../../scripts/oauth2/README.md).
+For more details on testing, see the [OAuth2 test scripts README](https://github.com/coder/coder/blob/main/scripts/oauth2/README.md).
 
 ## Common Issues