ci: reuse the release-branch container for CI:docs on release PRs

[yuki-97]

What does this PR do ?

CI:docs on PRs to a release branch now pulls that branch's container instead of :main. CI-only, no runtime change.

What changed

1. Extend the release-branch image tag to docs. #4616 switched CI:Lfast to :<branch> but left docs on :main, whose TRT-LLM wheel is keyed to main's ref. Docs_Tests on #4689 and #4692 (base r0.8.0, CI:docs) each spent ~85 min in Building tensorrt-llm before the doctests ran (~95 min job total). Only the IMAGE_TAG selection in cicd-main.yml changes; dispatch runs and main-based PRs still use :main.

2. Docs. docs/ci-cd.md, the cicd skill table, and the workflow_dispatch description now say docs follows Lfast.

Tests

CI:docs run on this PR (Docs_Tests):

	#4689 (before)	this PR
image	rl:main	rl:r0.8.0
Building tensorrt-llm	~85 min	4 s
Docs_Tests	~95 min	12 min

🤖 Generated with Claude Code

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.

[yuki-97]

/ok to test 583bf14