Support publishing GPL sources in workflows (#172)

* actions: collect-gpl-sources: add

Add a custom action for pulling GPL sources which we need to provide
access to for certain package builds. The canonical example is NumPy,
which we redistribute the gcc source used to build the wheels (and is
therefore the default). Since we expect this to be used with manylinux
container images (based on RockyLinux), use dnf-based tooling in the
action's steps.

* actions: publish-wheels: support publishing GPL sources

Publish GPL sources alongside our wheels if they're present so we can be
license compliant.

* workflows: build-numpy: add gpl_sources job

We need to ensure that the GPL sources used in the NumPy builds are
provided to be license compliant. Add a new job to get them and pass
them along to the publish step. Use a new MANYLINUX_RISCV64_IMAGE
variable at the environment level so that the collect-gpl-sources action
pulls the sources from the same container image we used for building.

* ci_scripts: update_doc.py: support GPL sources linking

Add some extra logic to automate linking to any GPL source artifacts we
provide along with our builds.

* docs: development.md: Add a 'Publishing GPL Sources' Section

* docs: development.md: fix heading level

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AI-Generated: Uses Claude Sonnet 5

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>