Update Deadline Docs to match deadline addon 0.5.0 (next release of ayon-deadline)

[MustafaJafar]

Is there an existing issue for this?

• I have searched the existing issues.

Things to add

• Custom Deadline Plugins: update this section and add info for each custom plugin

• **Explain multiple Deadline URLs: **
  Q: Can somebody shed some light for me on how "multiple deadline servers" work for ayon-deadline. I understand we can set multiple URLs at ayon+settings://deadline/deadline_urls but I'm clueless as to what actually defines which of the URLs is to be used for an instance?

  A: the worflow should be to define multiple Deadline servers and than pick them on a project level. It was a long requested feature as it was not possible to safely test render farm things in a separate environment from production.

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Additional context:

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