chore(infra): only build 0.3 ref docs from `v0.3` branches (#33322)

Using the `api_doc_build.yml` workflow will now only pull from the
`v0.3` branch for each `langchain-ai` repo used during the build
process. This ensures that upcoming updates to the `master`/`main`
branch for each repo won't affect the v0.3 reference docs if/when they
are re-built or updated.