Add Doxygen documentation infrastructure

[glopesdev]

• Doxygen version 1.12.0 and the Doxygen Awesome CSS theme were used.
• Root favicon and logo added for consistency with other Harp documentation websites.
• Triple slash added to existing function documentation comments so it gets automatically picked up by Doxygen.
• OPTIMIZE_OUTPUT_FOR_C flag was set to adjust generated docs terminology.

Following merge to main the new layout will be published.

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Documentation will automatically publish to GitHub Pages when:

• A release is made
• A manual workflow dispatch is initiated and publishing is enabled
• Any push to any branch is made when CONTINUOUS_DOCUMENTATION is enabled (intended for use in forks for development purposes, but this is not enforced.)

Preview at: https://glopesdev.github.io/core.atxmega/

[PathogenDavid]

For the benefit of anyone else who happens to notice that the globals index on the ATxmega core isn't super great compared to the same page in the Pico core:

Doxygen has a hard-coded cutoff where it splits those indices across multiple files and it's absurdly low at just 200 members. (This limit was set back in 2005 when dinosaurs still roamed the earth.)

Also worth noting that ALPHABETICAL_INDEX is completely unrelated to these pages. That corresponds to this index instead. (Quite annoyingly, this file doesn't seem to have a limit and even intelligently puts _ at the end.)

There's actually an issue and corresponding PR to improve this, but both seem to have stalled out. (To my dismay the PR unfortunately also doesn't let you disable the feature entirely and caps the maximum at 3,000.)

[PathogenDavid]

Everything I mentioned in harp-tech/core.pico#56 (review) also applies to this PR.