Fix stable/ links in readmes for moved pages

[GregoryComer]

📚 The doc issue

With the new documentation flow, I've moved, renamed, or split out a number of docs. There are hard-coded links in various readme files in the ET repo to stable/ docs. These are tricky to handle, because we can't easily relative link from readmes in the repo to docs (it likely should point to stable). So if we point to the new locations, it's broken now. If we don't, it breaks on the next release (when the new docs are promoted to stable).

To solve this, I see two options:

1. Fix the links during the 0.6 release process.
2. Set up redirects to handle the main moved pages (getting-started-setup being the most common one).

I'm leaning towards (2), then fixing the links permanently after 0.6 hits stable.

Suggest a potential alternative/fix

No response

cc @mergennachin @byjlw

[GregoryComer]

#10221 is merged. I'm cherry-picking this onto the 0.6 release branch and can close this out once that lands.