[Serve] [Docs] Remove references to namespaces in the Serve documentation

[shrekris-anyscale]

Why are these changes needed?

#25575 starts all Serve actors in the "serve" namespace. This change updates the Serve documentation to remove now-outdated explanations about namespaces and to specify that all Serve actors start in the "serve" namespace.

Related issue number

N/A

Checks

• 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 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
  • N/A: This is a documentation change.