Update theming for ownCloud Web

[mmattel]

WHAT Needs to be Documented?

As discussed with @kulmann, we need to document theming for owncloud web in our official web documentation. Same as we have done for config.json, we need to do for themes.json. Content source: https://owncloud.dev/clients/web/theming/
Note that theming is not dependent on the backend!

Complexity: 2

WHERE Does This Need To Be Documented (Link)?

A new themening section needs to be added.

WHY Should This Change Be Made?

Completeness of docs

(Optional) What Type Of Content Change Is This?

• New Content Addition
• Old Content Deprecation
• Existing Content Simplification
• Bug Fix to Existing Content

[mmattel]

https://doc.owncloud.com/ocis/next/deployment/webui/webui-theming.html

[kulmann]

To clarify, this only has two action items:

• copy the content for ocis 4.0
• link to the dev docs for ocis 5.0

No additional content needed.