Add multiversion support

[dalito]

This pull request introduces several improvements to documentation deployment, schema versioning, and developer experience. The most significant changes include enhanced multi-version documentation support and dynamic schema versioning.

Documentation deployment and multi-version support:

• Updated .github/workflows/deploy-docs.yaml to deploy documentation on pushes to main, and (new) on version; added versioned deployments using mike.
• Added mike plugin and configuration to mkdocs.yml for multi-version documentation, including version selector and canonical version support.
• Improved theme customization in mkdocs.yml and added light/dark mode logo support with corresponding HTML and CSS overrides.

Schema versioning and documentation:

• Introduced dynamic versioning for the schema using uv-dynamic-versioning and hatch build hooks in pyproject.toml; schema files are now auto-versioned and referenced in documentation.
• Added a new docs/versioning.md page explaining the SchemaVer scheme and how schema versions are managed and tagged.
• Updated navigation and references to point to the new elements/overview.md for schema documentation.

Developer experience and testing:

• Added coverage to development dependencies in pyproject.toml, and configured coverage reporting and source paths.
• Added a coverage recipe to project.justfile for running tests and generating coverage reports.

Other improvements:

• Added a README to docs/schema/ explaining the merged model and versioning of schema files.

Closes #20
Closes #13
Closes #9

[dalito]

I am going to merge this later today (~8 pm CET).

I will add copying of the dcat_ap_linkml.yaml file to gh-pages in a follow-up PR. This is already big enough.