Deprecate the label extension

[gadomski]

Related Issue(s):

• Closes Deprecate the label extension #1249

Description:

I kept the hooks in place (__init__.py) because the label extension adds a new object link rel type, "source". Removing that hook would change behavior, so we keep it until we make a breaking change to remove the extension altogether.

I also removed the label extension from all tutorials, and added some notebook-required dependencies to the doc optional dependency group.

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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files	Coverage Δ
pystac/__init__.py	94.91% <100.00%> (+0.27%)	⬆️

... and 2 files with indirect coverage changes

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[jsignell]

I love this