Unified `gh-pages` creation

[Bullrich]

Following @mordamax's advice to use workflow_call event, I ended up building a chain action.

The action publish-gh-pages only runs on the master branch and it will trigger the docs actions.

After it is done, it will fetch all the artifacts it uploaded and will use them to update the gh-pages branch.

Resolves paritytech/ci_cd#1026