ci: add docs-dispatch workflow (composer pings)

[makegov-hal]

Summary

• Adds .github/workflows/docs-dispatch.yml — on push to main, pings makegov/docs via repository_dispatch when docs/, README.md, or CHANGELOG.md changes.
• Keeps the public docs site in sync without anyone needing to push to the composer manually.

Why a separate PR

This is the editorial-split / composer wiring. Splitting it from the initial commit so it's easy to see, easy to revert if we ever change the docs flow, and consistent with how the sibling SDKs (tango-node, tango-python) ship the same workflow today.

Setup checklist (do before merging)

• Create a fine-grained PAT or app token with contents:write on makegov/docs.
• Save it as repo secret DOCS_DISPATCH_TOKEN on makegov/tango-go.
• (Optional) Set repo variable DOCS_TARGET_REPO if dispatching anywhere other than makegov/docs.

Testing

• After merge, touch docs/CLIENT.md (or any tracked doc file), push to main, confirm the dispatch fires in Actions logs.
• Confirm makegov/docs sees the external_updated event and rebuilds.

Risks

• If DOCS_DISPATCH_TOKEN isn't set, the dispatch step fails loudly — the workflow doesn't affect anything else. Worst case: docs site lags until someone pushes manually to the composer.