Add available config customization hooks documentation

[BigRoy]

Issue

What is available for a studio configuration to customize is still greatly undocumented. Which will mean many won't know about all the features and might be missing out on what they are actually looking for.

These customizations should be described, documented and some examples should be provided.

What to add

• Describe config.install() and config.uninstall() and explain what to put there.
  • Set up global plug-in paths for Loaders, Creators, Pyblish. Example
• Describe config.{host}.install() and config.{host}.uninstall() and what to put there per host integration.
  • Set up host-specific plug-in paths for Loaders, Creators, Pyblish. Example
  • Register host specific callbacks. Example
  • Also the perfect place to register Pyblish callbacks. Example
  • Set the default state for family filters in the Loader. Example. This way you can hide families by default that irrelevant to a specific host. The filtering can also be reversed so the list you pass is an "inclusive" default as opposed to an "exclusive". Example
• Describe config.{host}.collect_container_metadata and how this can be used to "parse additional data" for containers returned by host.ls() where for example an additional color value could be added to customize the colors in the Scene Inventory. I believe @davidlatwe had some production use cases for it too.
• Describe config.register_launcher_actions which is used in the Launcher to register actions from the studio config. Example which imports this function

There might be more that I missed now - exactly the reason why they should be documented.