ci(docs-dispatch): authenticate as the dispatchbot GitHub App

[makegov-mark]

What

Switches .github/workflows/docs-dispatch.yml from the retired DOCS_DISPATCH_TOKEN PAT to the makegov-dispatchbot GitHub App.

• A new actions/create-github-app-token@v3 step mints an installation token from the DISPATCHBOT_APP_ID / DISPATCHBOT_PRIVATE_KEY org secrets, scoped to the one repo the dispatch hits, and runs under the same condition as the dispatch step.
• A small resolve step derives that repo from DOCS_TARGET_REPO (default makegov/docs), so the token scope and the dispatch target cannot drift apart when the variable overrides the target.
• The dispatch step reads GH_TOKEN from that step's output through env:.
• The header's "Required secrets" comment names the two org secrets. Change detection is untouched.

Testing

• actionlint reports only the existing SC2063 note on the grep -qx '0*' all-zeros check, which is intended.
• A manual workflow_dispatch run on this branch, which always dispatches, mints the token and sends the dispatch end to end.
• The workflow matches the other SDK repos' copy apart from its header comment and checkout version.

Notes

CI-only change with no effect on the published package, so no changelog entry. [skip changelog]

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