[Docs] Update TPU Documentation

[ryanaoleary]

Why are these changes needed?

This PR contains some small edits to the TPU Ray documentation to clarify the sections, specifically making it more clear that the Ray TPU initialization webhook is included in the GKE Ray addon and manual installation is optional.

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• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[andrewsykim]

some minor comments, LGTM otherwise

[kevin85421]

I have already pinged Ray doc team to review.

[dayshah]

lgtm from docs perspective

[ryanaoleary]

is someone with write access able to merge this for me? @dayshah @kevin85421