Fix/576 AssetDefinition missing create, apply

[guidorice]

Related Issue(s): #

Fixes #576

Description:

• Adds missing create and apply to Asset Definition
• Enhances an unrelated unit test, which I noticed while comparing other code (test_eo.py)
• Fixes old commonmark version in some docstrings.
• Fixes docstrings referencing RC of stac spec.

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• 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.

[codecov-commenter]

Codecov Report

Merging #768 (f3ab90d) into main (a19365b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #768   +/-   ##
=======================================
  Coverage   94.54%   94.55%           
=======================================
  Files          79       79           
  Lines       11626    11647   +21     
  Branches     1372     1372           
=======================================
+ Hits        10992    11013   +21     
  Misses        455      455           
  Partials      179      179           

Impacted Files	Coverage Δ
pystac/catalog.py	92.15% <ø> (ø)
pystac/collection.py	94.26% <ø> (ø)
pystac/extensions/item_assets.py	86.00% <100.00%> (+1.55%)	⬆️
tests/extensions/test_eo.py	100.00% <100.00%> (ø)
tests/extensions/test_item_assets.py	100.00% <100.00%> (ø)

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[duckontheweb]

Thanks for this @guidorice, and thanks for catching some of the outdated docstrings. Always good to have a new set of eyes on this stuff!