"Create a custom provider for Companion" guide

[Murderlon]

The Companion docs are already quite big and this topic deserves some love. We should make a guide for it and link to it from the Companion docs.

[Murderlon]

Here is an initial content outline. I was thinking since we likely will not merge webdav, but the PoC is already written, it's perfect to copy paste the implementation bits over on how someone would implement something like webdav.

Outline:

• Custom providers
  • What are custom providers
  • What kinds are there (auth, no auth, search)
• Example: building a webdav provider
  • what is webdav
  • server-side implementation
  • client-side implementation
• FAQ