ci: eliminate duplicate docs builds

[abrichr]

Summary

• avoid running the generic CI build and the deploy workflow for the same docs-source push
• remove the docs repository's active copy of the cross-repository dispatch template, which dispatched the repository back to itself
• document that content repositories own the narrow notify-docs.yml dispatch

Why

sync.yml already installs, tests, validates, and builds immediately before deployment. Running ci.yml for the same docs-only push duplicated that work. The deleted workflow explicitly described itself as a template for other repositories, but because it lived under .github/workflows here, GitHub executed it on every main push.

Validation

• actionlint .github/workflows/ci.yml .github/workflows/sync.yml
• git diff --check

Tooling/code changes still run generic CI, and every deploy path still validates before publication.