docs: add versioning with mike

[michalsn]

Description
This PR adds docs versioning using mike, published to the gh-pages branch.

Deploy rules

• Push to develop: deploys the dev docs (work-in-progress)
• Push a vX.Y.Z tag (e.g. v2.4.0): deploys docs under <major>.<minor> (e.g. 2.4), updates the latest alias, and sets it as the default version (site root redirects here)
• Pre-release tags (e.g. v2.4.0-rc1): are skipped

Closes #165

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide