docs: generate version-scoped llms.txt artifacts (#2188)

Close #2185. This PR replaces the root-level LLM documentation artifacts with
version-scoped outputs:

- `/v1/llms.txt`
- `/v1/llms-full.txt`
- `/v2/llms.txt`
- `/v2/llms-full.txt`

The goal is to avoid mixing AgentScope Java 1.x and 2.x documentation in
a single `llms-full.txt`, so AI coding tools can ingest the correct
documentation set for the target version.