Update docs to reflect that collection rel type can be for anything

[gadomski]

Related Issue(s):

• Closes Add collection as a relation type #1379

Description:

We already have a collection rel type, so this PR is just a small docs tweak to reflect that collection links don't have to be only for items.

It's not required, but it would be nice to move {get|set}_collection to STACObject from Item:

pystac/pystac/item.py

Lines 260 to 295 in 28352e7

	def set_collection(self, collection: Collection | None) -> Item:
	"""Set the collection of this item.
	This method will replace any existing Collection link and attribute for
	this item.
	Args:
	collection : The collection to set as this
	item's collection. If None, will clear the collection.
	Returns:
	Item: self
	"""
	self.remove_links(pystac.RelType.COLLECTION)
	self.collection_id = None
	if collection is not None:
	self.add_link(Link.collection(collection))
	self.collection_id = collection.id
	return self
	def get_collection(self) -> Collection | None:
	"""Gets the collection of this item, if one exists.
	Returns:
	Collection or None: If this item belongs to a collection, returns
	a reference to the collection. Otherwise returns None.
	"""
	collection_link = self.get_single_link(pystac.RelType.COLLECTION)
	if collection_link is None:
	return None
	else:
	return cast(
	Collection, collection_link.resolve_stac_object(self.get_root()).target
	)

However, as documented in #1392, set_collection returns self, which makes it a bit more awkward to return from the STACObject base class. My instinct is to hold off for moving {get|set}_collection, and just create a tracking issue in the v2.0 milestone so that we can solve #1392 and the method move at the same time.

PR Checklist:

• pre-commit hooks pass locally
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.
• Create tracking issue for moving {get|set}_collection to STACObject: Move {set|get}_collection to STACObject #1399

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.19%. Comparing base (e326b41) to head (6541a83).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1393   +/-   ##
=======================================
  Coverage   91.19%   91.19%           
=======================================
  Files          52       52           
  Lines        7238     7238           
  Branches     1019     1019           
=======================================
  Hits         6601     6601           
  Misses        457      457           
  Partials      180      180           

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[jsignell]

lgtm