Add docs for runtime theming

Description

Add docs for runtime theming

Supporting information

https://tasks.opencraft.com/browse/BB-8502

Testing instructions

NA

Dependencies

• opencraft/documentation/tech!11 (merged)
• tutor-contrib-grove!70 (merged)

Checklist

If any of the items below is not applicable, do not remove them, but put a check in it.

• All providers include the new feature/change
• All affected providers can provision new clusters
• Unit tests are added/updated
• Documentation is added/updated
• The TOOLS_CONTAINER_IMAGE_VERSION in ci_vars.yml is updated
• The grove-template repository is updated