fix(docs): correct rustdoc merge and add /py-rattler/ redirect so docs URLs work

[Anshgrover23]

Description

Fixes two docs deployment issues on rattler.prefix.dev.

1. /py-rattler/ had no page because docs were only deployed under /py-rattler/latest/ and /py-rattler/dev/.
   A redirect is added so https://rattler.prefix.dev/py-rattler/ redirects to https://rattler.prefix.dev/py-rattler/latest/.

2. Rust docs were previously deployed with an extra nested directory at /rattler/rattler/.
   The deploy step now places the Rustdoc output directly in the correct location.
   The documentation is now served at https://rattler.prefix.dev/rattler/index.html.

How Has This Been Tested?

• Checked the workflow YAML and the copy/redirect steps.
• Deployed from a fork to GitHub Pages and confirmed:
  • Python docs (latest): https://anshgrover23.github.io/rattler/py-rattler/latest/
  • Rust docs: https://anshgrover23.github.io/rattler/rattler/index.html

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes

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Note for maintainers: After merging, the fix will apply the next time the docs workflow runs (e.g. on push to main or when the workflow is triggered). For /py-rattler/ to successfully redirect to latest/, you’ll need to do a new release so the release docs job runs and deploys the latest Python docs to gh-pages.

[Anshgrover23]

@baszalmstra Could you review this one when you get a chance?