Migrated `docs` scripts to GitHub actions (#5345)

Migrated the following scripts to GHA
- test-doc
- test-rustdoc
- build-rustdoc
- build-implementers-guide
- publish-rustdoc (only runs when `master` is modified)

Resolves paritytech/ci_cd#1016

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Some questions I have:
- Should I remove the equivalent scripts from the `gitlab-ci` files?

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Co-authored-by: Alexander Samusev <[email protected]>