Support multiple versions in the website documentation

The documentation is currently updated prior to a minor release with the supported functionality, thus losing the information of what is supported on each version. The immediate impact is that documentation cannot be written unless the functionality is completed because it needs to be deployed only after the release.

The support of multiple versions within the documentation would allow a smoother evolution of the documentation during development where a “in development” meta version can be updated for each individual functionality added to the system.

General requirements:

• the documentation must remain at https://bluebrainnexus.io/docs without affecting the product website
• it must support markdown with html customisations
• it must be searchable
• it must allow checking references
• it must support multiple versions (with navigation between version)
• it must be controlled through CI (deployment from the master branch)
• optionally: it must notify users that are browsing an older version of the documentation to navigate to the latest released version

[imsdu]

What an older version can look like:

[imsdu]

Completed when merging #2555