ci: Make regenerated-models comment on docs PRs more visible (#748)

## Summary

The `manual_regenerate_models` workflow posts a cross-repo comment on
the source `apify-docs` PR whenever a companion `apify-client-python` PR
is opened or updated. Some authors have been ignoring it, which leaves
the Python client out of sync with the OpenAPI spec.

This PR makes the comment harder to miss:

- Uses a GitHub `[!IMPORTANT]` alert block so it renders as a
highlighted callout.
- Adds an `@mention` of the docs PR author (passed in via the new
`docs_pr_author` workflow input) so they get a direct notification.
- Rewrites the body with an explicit "Action required" headline and a
short action checklist (review & merge both PRs together, ping the
Tooling team if needed).
- Minor cleanup: comment/docstring wording in the workflow file.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>