docs(manual): doc generator and drift checks, the chapter template, and the setup-environment chapter

[quantecon-services]

PR 2 of the user manual (#178). The design is settled in #178, decisions 3 and 5; this issue is the unit of work.

• scripts/generate-docs.py (Python 3 + PyYAML):
  • generates each chapter's Inputs and Outputs tables, each signpost README, and the action table in docs/user/README.md, between <!-- BEGIN GENERATED: … --> / <!-- END GENERATED --> markers;
  • --check prints the diff and fails.
• PyYAML pinned in a requirements file, with a pip entry in .github/dependabot.yml.
• actionlint installed as a pinned release with a checksum. It checks templates/*.yml and every complete workflow in a yaml block under docs/user.
• Input checks: every quantecon/actions/<x>@… step in the docs and templates uses only inputs that exist and supplies every required one. Every complete example has a permissions: block.
• All checks run in the harness gate job, beside the template pin-drift check from fix(actions): #107 correctness batch and #109 code items #173.
• The chapter template.
• docs/user/actions/setup-environment.md and its generated signpost README.

Done when the checks pass in the gate and the setup-environment chapter is merged.