Feature/issue 99 version selector

[PhilippSalvisberg]

Closes #99 - Add version selector in HTML site

• The solutions is based on mike 0.5.5 and Material for MkDoc 6.2.8. The integration is not super optimal, but this should be addressed as soon as Material for MkDocs comes with integrated mike support
• The HTML sites for v3.3, v3.4, v3.5, v3.6, v4.0 have been migrated manually (I added the CSS and JS changes via search & replace in all HTML files).
• In mike 0.6.0 Material for MkDocs CSS and JS patches are removed. The reason is that Material for MkDocs supports versions via mike directly. This feature is available since Material for MkDocs Insiders 1.7.0.
• According to the funding goals I expect versioning to be available very soon in the free version of Material for MkDocs. Then we can and should use this built-in versioning capability for new HTML sites (there should be no need to change already deployed site versions)
• I'd like to keep using the open sourced variant for Material for MkDocs, because it makes it easier to fork the repo and bild amended guidelines.