Versioned documentation

[Sija]

It would be great to have online documentation versioned, in order to be able to link to it in our tooling, since rules and rule options are subject to change between versions.

References:

• Add support for summaries in GitHubActionsFormatter #531 (comment)
• Add link to documentation vscode-crystal-ameba#146

[Sija]

Gameplan draft:

1. Remove everything from gh-pages branch
2. Commit versions.json file (see for example Crystal's one), containing a single nightly entry
3. Tweak the docs workflow by:
   • Adding --json-config-url=/ameba/versions.json argument to crystal docs invocation
   • Building the docs on every push to the master branch with target-folder: master
   • Building the docs on every release with target-folder: <version> & appending the version into the versions.json
4. Run the workflow for master & every published version so far
5. Create index.html in the root directory with redirect to /master

/cc @veelenga