Doc/affine units

[chikle9090]

This PR addresses issue #3902 by expanding the documentation for +proj=affine.

Summary of changes

• Clearly documents that affine input angular values are interpreted in radians.
• Clearly documents that affine output angular values are expressed in degrees.
• Adds explanation that proj +proj=affine cannot be used directly and will raise an error.
• Shows that affine must be used as a step in a pipeline.
• Adds examples demonstrating real behavior (such as the cs2cs example from issue Mention affine units #3902).
• Adds recommendations for correct usage when working with geographic vs projected coordinates.

Checklist

• Closes Mention affine units #3902
• Tests added (Not applicable: documentation-only change)
• Added a clear title that can be used to generate release notes
• Fully documented (updated affine.rst accordingly)

[hobu]

Is this LLM-generated code? I don't understand these changes or why they're grouped together the way they are.

The CMake changes should be their own PR if they're actually needed.

The install doc change is incorrect.

[chikle9090]

Closing this PR because it included unrelated commits.
I have opened a new clean PR with only the intended affine.rst documentation changes for #3902.