Support SVG switch elements

[dannymaaz]

Summary

• add basic SVG <switch> support
• select the first child without conditional processing attributes
• skip environment-dependent alternatives so an unconditional fallback can render
• update the SVG support documentation

Why

<switch> was ignored by the SVG parser, causing SVGs that use fallback graphics to render blank in that section.

Validation

• ruff check fpdf/svg.py test/svg/test_svg.py
• python -m pytest test/svg/test_svg.py -q (276 passed)

Checklist

• A unit test covers the change.
• Documentation and changelog entries are included.
• This pull request is ready for review.

By submitting this pull request, I confirm that my contribution is made under the terms of the GNU LGPL 3.0 license.

[andersonhc]

Thank you very much for the contribution @dannymaaz
I'm good to merge the PR

[andersonhc]

@allcontributors please add @dannymaaz for code

[allcontributors]

@andersonhc

I've put up a pull request to add @dannymaaz! 🎉