[Doc][namespaces][C++ worker]add document for c++ worker namespace and specifying namespace while creating/getting named actors

[WangTaoTheTonic]

Why are these changes needed?

We've supported namespace in c++ worker in #26327. Here we add doc for usage and also reinforce the documents of Java and Python, like adding explanation of specifying namespace while creating named actors.

• add doc for basic c++ worker namespace usage
• add explanation for specifying namespace while creating named actors, in Python, Java and C++

Related issue number

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
  • Unit tests
  • Release tests
  • This PR is not tested :(

[WangTaoTheTonic]

@edoakes @ericl @maxpumperla @pcmoritz @richardliaw @simon-mo
Hi, could you help to review this doc change? Thanks.

[jovany-wang]

@WangTaoTheTonic You missed DCO.

[jovany-wang]

@ericl @simon-mo This PR needs to be approved by doc code owners, please take a look, thanks.

[simon-mo]

@scv119 please take a look; since this is related to Ray Core documentation

[simon-mo]

10001 is the ray client port; i believe it's not recommended in python and not supported in Java and C++. this means the Java and CPP code will fail

[WangTaoTheTonic]

Oh I didn't know it's not recommended in python. I'll change the Java and C++ part first.

[WangTaoTheTonic]

I've removed the address part in C++. For the python part I'd like to keep it consistent with other descriptions in same doc. What do you think? @simon-mo

[simon-mo]

hmm for these part and consistency issue, @scv119 and @jjyao should be able to take a look and recommend the best course of action here as they are also thinking about refactoring the documentation here.