Build documentation with PasDoc and deploy to GitHub Pages

[suve]

Since our original discussion in #28, GitHub has added support (currently in beta) for deploying stuff to GitHub Pages from GitHub Actions. This PR adds a workflow that builds documentation using PasDoc and deploys it to GHPages.

One thing to consider: this introduces a new workflow, which is completely independent from the existing build/test workflow. As such, the docs will get build even if the build/test workflow fails. While it's not exactly straight-forward, it is possible to create inter-workflow dependencies in GHActions. The question is, then: do we want the docs workflow to require the build/test workflow to succeed in order to actually launch?

[Free-Pascal-meets-SDL-Website]

Due to lack of time to review, I will come back to this soon. But I'm happy to see this going forward.

[Free-Pascal-meets-SDL-Website]

To be honest, I have zero experience with GH pages.

The script looks good, though I wonder if we need to set up a "project site" prior to deploying the compiled docs? How does GH pages know that the compiled docs are in gh-pages folder? Do we need to set the page up first?

https://pages.github.com/ (click on "Project site" and "Start from scratch")

[suve]

The script looks good, though I wonder if we need to set up a "project site" prior to deploying the compiled docs? (https://pages.github.com/ -> click on "Project site" and "Start from scratch")

Reading through the page, it seems to describe the "deploy from a branch" workflow, not the "deploy from GitHub Actions" one. Which makes sense, since the latter is still marked as "beta".

How does GH pages know that the compiled docs are in gh-pages folder?

I assume the Pages-related actions take care of that. I mostly just copied from GitHub's example workflow.

Do we need to set the page up first?

As far as I can tell, all we need to do is go to the repository's settings, and in the Actions tab, choose "GitHub Actions" as the deployment source. (Already done this.)

I played with this shortly in my fork and the deploy went ok. Reverting the Pages site to an older version, or taking it down altogether is also pretty straightforward - so should we, at some point in the future, decide that the "build and maintain docs" idea was a mistake, it's easily fixable.

[Free-Pascal-meets-SDL-Website]

I see, so let's see what happens! Merged!

EDIT: Congratualtions! It works like a charm!

[suve]

The CI run completed successfully, the built docs are now available at pascalgamedevelopment.github.io/SDL2-for-Pascal/.

[Free-Pascal-meets-SDL-Website]

Congratulations again. The link should be added to the README.md.