CI: add Doxygen documentation generation testing

[viniciusferrao]

Description

The CI container currently skips Doxygen because it's not available in UBI 10 repos or EPEL 10. The docs and doxygen-xml CMake targets are never tested in CI.

Current blockers

• Doxygen not in UBI 10 repos: The doxygen package exists in full RHEL 10 CRB (doxygen-1.13.2-1.el10) but Red Hat does not include it in the UBI 10 CRB subset (which only has ~54 packages).
• Doxygen not in EPEL 10: No epel10 branch exists in the Fedora doxygen src RPM, so EPEL doesn't carry it either.
• Devcontainer works: Local devcontainer builds on subscribed RHEL hosts get doxygen from the full CRB repo. Only CI (UBI-based) is affected.

Possible solutions

1. Install AlmaLinux CRB RPM directly: dnf install -y https://repo.almalinux.org/almalinux/10/CRB/x86_64/os/Packages/doxygen-1.13.2-1.el10.x86_64.rpm — binary compatible with UBI 10, one-line fix.
2. Build Doxygen from source: Similar to the Bloaty build-from-source approach already in the Containerfile.
3. Separate docs workflow: Run docs generation in a dedicated workflow using a different base image (e.g., AlmaLinux 10 or Fedora).
4. Wait for EPEL 10: Monitor if doxygen gets added to EPEL 10 in the future.

Acceptance criteria

• Doxygen installed in CI container
• doxygen-xml target built and validated in CI
• docs target (Sphinx + Breathe) tested if applicable