[Serve][Doc] Add Batching User Guide

[simon-mo]

Why are these changes needed?

Add a new page discussing how to use the batching decorator.

Related issue number

Closes #27445

Checks

• [x] I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• [x] I've run scripts/format.sh to lint the changes in this PR.
• [x] I've included any doc changes needed for https://docs.ray.io/en/master/.
• [x] 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
  • [x] Unit tests
  • [ ] Release tests
  • [ ] This PR is not tested :(

[edoakes]

Will do a pass after @dmatrix's comments are addressed

[simon-mo]

Comments addressed! I think we can keep the tutorial example which uses PyTorch and show case how to compile batches into tensors and others.

[edoakes]

@simon-mo you need codeowner approval from someone here