[amol-]

Replace GitHub-event signing with Read the Docs’ documented Generic API integration:
https://docs.readthedocs.com/platform/stable/guides/setup/git-repo-manual.html#webhook-integration-generic

• Send a form POST with the repository-secret token, branches=master, and default_branch=master.
• Remove the Python signing code and pending diagnostic expansion.
• Preserve step-scoped credentials, canonical master-push restrictions, and all existing runtime and HTML/PDF checks.
• Fail on HTTP errors or a receipt that does not confirm a build was triggered.

Validation: curl boundary tests cover form encoding, request parameters, credential privacy, HTTP/network/timeout failures, and receipt handling. Gating, YAML/Bash/actionlint and whitespace checks passed. Code-quality review approved the corrected request.

Verified after merge:

• All eight master-push CI jobs passed, including the Generic API publishing step: https://github.com/bbangert/beaker/actions/runs/41142646488
• RTD successfully built latest at merge commit ef06437: https://app.readthedocs.org/projects/beaker/builds/35536378/
• Published HTML and the readable 61-page PDF show Beaker 1.14.1. Documentation uses the single-version URL https://beaker.readthedocs.io/.