ci: Allow manually dispatching the master Checks workflow (#911)

## Summary

Adds `workflow_dispatch` to `CI (master)` so the `Checks` suite can be
re-run manually on the same commit when the push-triggered run is lost —
e.g. during a GitHub outage. The `doc_release` / `beta_release` jobs are
gated on `github.event_name == 'push'` so manual dispatches only re-run
the checks and never re-publish.

The redundant `workflow_dispatch` trigger on `_checks.yaml` is removed:
dispatching it directly produced check-run names without the `Checks /`
prefix, so the `wait-for-checks` gate in the manual release workflows
wouldn't recognize them anyway.

Mirrors apify/crawlee-python#1917.

## How to use

When a push-triggered Checks run on `master` is missing, go to **Actions
→ CI (master) → Run workflow** on `master`. Once it succeeds, the manual
release workflows' `wait-for-checks` gate will be satisfied.