docs: Update docs in core and gen

[yamaceay]

What this PR does and why it is needed

This PR aims to fix the inconsistencies of (1) docstring of AICoreV2Client.from_env method and (2) the authentication section of packages/gen/gen_ai_hub/docs/README.md. The changes are related to the closed SAP/ai-sdk-python-backlog#1.

Definition of Done

• Documentation updated
  • Only Public APIs are allowed to be used in documentation/tutorials/sample code

[alpkom]

I've missed your previous PR for actually implementing this. I've just checked and it looks like you implemented the precedence of credentials different than the way it's handled in LiteLLM. Since the LiteLLM one is implemented first, and this is not documented yet, Let's first align the precedence and then document it.
The precedence should be:
kwargs → service key → env (AICORE_*) → config → VCAP service.

@yamaceay could you please adjust the implementation first and then document it accordingly?

[mwien]

lgtm

[alpkom]

Looks good to me.

But can you add a dummy commit with "feat(core): add support for service key" in the title?
The reason is, this change is part of core, and we'll need to create a new version of core. However, unless there is a commit with title "feat/fix(core)" the commitizen (the tool we use for version control) won't pick it up and assume no release-relevant changes were done for core.