Add doxygen docs to Github Pages

[yohannd1]

This PR adds a github workflow that runs doxygen on the codebase and uploads the generated HTML pages to Github Pages. My aim with this is to make reading the docs easier and browsing through the documentation simpler.

This workflow was based off the build workflow already present in the repo, plus some prior Github Pages experiments I've worked on.

As the workflow depends on access to Github Pages, it's not working in the PR. Meanwhile, you can check it on my fork and its github pages instance.

Things to consider:

• Where will this be hosted? I think in the current state it would go to https://lmms.github.io/lmms.

[yohannd1]

Note that the CI run inside the PR does not work fully, as it is denied acess to github pages. Not sure if there's a way to work around that. In fact, I'll be disabling this run on PRs for now.

[messmerd]

Following the advice from this Stack Overflow article, I just set the GitHub Pages source for this repo to GitHub Actions.

Try again and see if it works now.

[yohannd1]

That seems to not have worked - it did go farther, but it still has permission-related errors:

I will now be trying rossjrw/pr-preview-action, which might be able to solve this.

[messmerd]

I will now be trying rossjrw/pr-preview-action, which might be able to solve this.

I don't think that will work, unless we set things up differently here.

From that action's readme:

[yohannd1]

Ah, I see. From what I've read here it seems it is possible to adapt the workflow here, but I'm not sure if it would be worth it - pushing tons of generated HTML and graph files of tons of PRs is probably going to bloat the repo a lot.

[JohannesLorenz]

What I checked:

• Code
• Style

[rubiefawn]

I did some local testing and it looks like we can get rid of the old relative path hacks by just using @CMAKE_SOURCE_DIR@ instead.

[yohannd1]

Oh, nice. Do I apply the changes here? I see you've done something similar but slightly different in #8361.

[rubiefawn]

I'll be rebasing #8361 once this PR is merged anyways, so I suppose it doesn't matter that much lol

(The "local testing" got out of control and ended up as that other PR)

[rubiefawn]

What, if anything, does this PR need before it's ready for merge?

[yohannd1]

I don't think there is anything pending here. I'd like to make a detailed initial page guiding through the codebase but that should go into a separate PR.