Fix and clean up documentation build by using @extref

[kahaaga]

The documentation build in #400 was failing due to cross-reference and external cross reference issues that I did not manage to debug. The solution was to completely re-do how cross-referencing is done in the docs, according to the standards in the latest versions of Documenter.jl.

• The documentation now is completely stand-alone. It does not build the documentation from other packages. Previously, the docs for Associations.jl also built part of the docs for ComplexityMeasures.jl and StateSpaceSets.jl, which caused cross-reference issues and dead links with the latest versions of Documenter.jl)
• All types and methods references in the docs belonging to other packages (e.g. ComplexityMeasures.jl) are now referenced using an explicit @extref (e.g. [`OutcomeSpace`](@extref ComplexityMeasures.OutcomeSpace)). This eliminates all documentation cross-reference errors.
• Docs are now shorter (arguably cleaner), because of the external references. I'd prefer to maintain this way of documenting external references going forward, so we don't need to do double maintenance when it comes to docs (i.e. keeping the docs manually up to date in Associations.jl AND updating the docs in ComplexityMeasures.jl).
• Un-collapse all docstrings by default (since many pages are now shorter)
• Use checkout action v5.
• Set minimum julia version to long term stable release (1.10.10).