ci: add docs-dispatch workflow (composer pings)

[makegov-hal]

Summary

• Adds .github/workflows/docs-dispatch.yml: on a push to main that changes docs/, README.md or CHANGELOG.md, it sends a repository_dispatch so the docs site at docs.makegov.com rebuilds.
• Same trigger paths, dispatch payload and actions/checkout@v6 as the workflow the Python and Node SDKs run.
• Hardened beyond the original sibling workflow (the siblings are getting the same change):
  • Step outputs and event fields reach the scripts through env: as quoted shell variables, never as ${{ }} expanded into the script text, so a file name can't inject shell.
  • Change detection diffs the whole push (github.event.before..github.sha, full history checked out) instead of only the last commit, so a multi-commit push can't miss an earlier docs change.
  • A manual workflow_dispatch, a new branch (all-zero before) or a force-push whose before is no longer in the history dispatches unconditionally, reporting the last commit's paths.

Before merging

• Add the DOCS_DISPATCH_TOKEN repository secret (a token with contents:write on the docs repository). Without it the dispatch step fails on every docs change.
• Optionally set the DOCS_TARGET_REPO repository variable to override the dispatch target.

🤖 Generated with Claude Code

~ Mark