doc: add versioning using "mike"

The Python tool 'mike'[1] helps manage multiple versions of MkDocs-powered
documentation.  Only required when building the kernelkit.org site docs!

For each new tag, that steps year or month, a new version of is created of
the documentation, in a separate sub-directory on the gh-pages branch.

Strategy - Version Grouping with Early Publishing:

- Extract YEAR.MONTH from tag
  E.g., v25.06.0-beta1, v25.06.0-rc1, v25.06.0, v25.06.1 → become version 25.06
- From first pre-release onwards:
  - v25.06.0-beta1 → creates docs version 25.06
  - v25.06.0-rc1 → updates docs version 25.06
  - v25.06.0 (GA) → updates docs version 25.06
  - v25.06.1 (patch) → updates docs version 25.06
- New major/minor series:
  - v25.07.0-beta1 → creates new docs version 25.07

Benefits:
 - Users get docs as soon as beta/rc is available
 - Patches update existing docs (logical since patches rarely change docs significantly)
 - Clean version grouping by YEAR.MONTH
 - Mike handles create vs update automatically

[1]: https://github.com/jimporter/mike

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

mkdocs.yml

@@ -110,7 +110,10 @@ markdown_extensions:
 plugins:
   - search
   - callouts
+  - mike
 
 extra:
   generator: false
   homepage: https://kernelkit.org/
+  version:
+    provider: mike