docs: reshape README Quick start, Usage, and types

[raulgg]

Summary

Rewrite the README how-to: Quick start is three inserts for an existing workflow, Usage is the assembled file plus reference (Inputs, Outputs, should-run, types). Types and should-run say what this action needs, not how GitHub webhooks work.

Why

stacked and edited sit outside GitHub's default pull_request activities. Copying on: pull_request with no types list never starts a run on gh stack link or when GitHub retargets the next PR after a bottom merge. The old README buried those facts and repeated YAML in two places.

Changes

• Quick start: types, always-run gate with pull-requests: read, skip from should-run.
• Usage: pin a major tag (@v1 in examples), merge_group, job-level permissions, pull_request vs pull_request_target, then the full workflow. No RELEASING.md. No patch tag in the pin line.
• Inputs, Outputs, How should-run is decided, and pull_request types are ### under Usage.
• Types table: why this action needs opened, synchronize, reopened, edited, stacked.
• Opening skip rule: remaining bottom, top if you leave it on, mid-stack skips.

Verification

• npm test
• Unit test for the change (file)
• Live path (opened / stacked / synchronize / remaining-bottom)