docs: fix two broken cross-references (#19296)

[ChrisJr404]

Summary

The Commands and Settings bullets in docs/reference/index.md use relative links (./cli.md and ./settings.md), but those files were removed from the repo in #16969 (they're now generated only at publish time). The links 404 when viewed on GitHub, and mkdocs build --strict emits two warnings for reference/index.md against a clean checkout.

This swaps the two links to absolute URLs, matching the pattern already used elsewhere in the docs (e.g., docs/concepts/cache.md, docs/concepts/projects/init.md).

Before / after

-- [Commands](./cli.md): A reference for uv's command line interface.
-- [Settings](./settings.md): A reference for uv's configuration schema.
+- [Commands](https://docs.astral.sh/uv/reference/cli/): A reference for uv's command line interface.
+- [Settings](https://docs.astral.sh/uv/reference/settings/): A reference for uv's configuration schema.

The other three bullets in the same list (./internals/resolver.md, ./storage.md, ./policies/index.md) target files that exist in the repo and continue to work both on GitHub and on the rendered site, so they are left as-is.

Test plan

• Confirmed both target URLs resolve on docs.astral.sh.
• uv run --only-group docs mkdocs build --strict -f mkdocs.yml: the two Doc file 'reference/index.md' contains a link './cli.md' / './settings.md' warnings are gone after this change.

Closes #19296.