Add redirects for relocated docs

[pytorchbot]

Summary

We've moved a number of doc pages in 0.6. This PR adds the sphinx-reredirects extension, which adds a sphinx feature to define redirected doc pages. I've used this to declare redirects for key relocated doc pages to preserve SEO and incoming external links.

Solves #8728.

Test plan

I've locally built and served the docs and navigated to each of the redirected URLs. I confirmed that all redirect as expected.

cc @mergennachin @byjlw

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/10241

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

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[metascroy]

Waiting for CI before merging