Add docs for how the _object_links method should be used in extensions

[lossyrob]

The _object_links method is important for extensions that provide new links to other STAC objects. This allows PySTAC to know what links point to other STAC objects for things like fully resolving a catalog into an in-memory structure.

Currently that method has little documentation around it, so new extension authors won't know to implement that method appropriately. The goal of this issue is to add docs such that someone implementing an extension would be able to successfully implement that method for any extension that adds links to a STAC object.

[lossyrob]

Closing as _object_links for extensions is going away with #309 and the new way of doing this (ExtensionHooks) should be documented as part of other issues.