Move development/contributing docs to docsy theme repo (embed via /docs/development), sync tools with other handbooks (hugolint, Hugo v0.123.7, CI/CD components)

Why is this change being made?

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The documentation in the README.md became the only updated location for handbook editing, bringing the internal and docsy-gitlab projects out of sync.

1. This MR moves the development documentation to the docsy-theme (it deletes all README.md content, re-added and revised in docsy-gitlab!72 (merged))
   1. Bump docsy-gitlab to v0.3.16 which includes the documentation updates, and pulls it into the handbook sources as a Go module on hugo build
2. CI/CD: Update hugolint to handbook-tools container registry path
3. Bump the required Hugo version in config.yml to be in sync with CI/CD and documentation
4. Rename LICENCE to LICENSE, fix copyright year to avoid future yearly updates
5. Use CI/CD components instead of templates
6. Bump docsy-gitlab to v0.3.17 to fetch the latest /docs/development content

Related to

1. Handbook: Get to fast, stable state (&10)
2. Development docs: General docs, Hugo module dep... (docsy-gitlab#4 - closed)

Author and Reviewer Checklist

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