Add support for Themes - oik-themes

[bobbingwide]

Full Site Editing (FSE) themes have started to appear on wordpress.org. I think it's time to start creating a catalogue of them.
Obviously the one that I know most about is Fizzie, but there are quite a few more I'm aware of. See /bobbingwide/oik-themes/issues/11.

Requirements

• Ability to discover themes
• Archive display of all FSE themes
• Display of individual themes
• Display of the templates and template parts
• Display of other useful information about the theme

Proposed solution

• Add Themes link to the main header menu - linking to oik-themes to display the archive
• Add an archive-oik-themes.html template
• Add a single-oik-themes template
• Make use of improvements to oik-themes plugin

[bobbingwide]

• The archive-oik-themes.html template would be based on archive-oik-plugins.
• The single-oik-themes.html template is based on single-oik-plugins.
• There will be some new template parts for A2z navigation and downloads
• The theme download link will need to cater for FSE themes being non-hosted.

[bobbingwide]

Delivered in v0.3.0

The theme download link will need to cater for FSE themes being non-hosted.

This was in oik-themes: see bobbingwide/oik-themes#11