ci: fix broken doc build & update GitHub Actions versions

[Ab2nour]

Context

As @arokem said in #116, doc build in CI is currently broken.

What's included

The new CI runs without errors: documentation build works (all unit tests still pass).

Changes include:

• Updated deprecated GitHub Actions versions (checkout, setup-python, upload-artifact)
• Replaced deprecated ACCESS_TOKEN with GITHUB_TOKEN
• Removed redundant pip install .
• Added sphinx.ext.githubpages Sphinx built-in extension, to ensure _static/ files (e.g. CSS) are properly served on GitHub Page (1)
• Removed indent in docs index.rst that caused HTML malformation (2)

(1) Explanation

This is for automatically adding the .nojekyll file.

Even though there is already one in the current gh-pages branch, if a new contributor wants to deploy the documentation in his own fork, the .nojekyll file will be absent, so I suggest this as a better practice for contributions.

Feel free to argue about this one.

(2) Explanation with screenshots

With the updated CI, the previous index.rst indent produced this result:

HTML malformation

Which is not expected, and not what's in the current doc online. So I fixed it.

Nitpicking: due to updated version, the design isn't exactly the same. I see no problem here though.

Current documentation (before) / documentation after CI update (after)

Preview

I uploaded the documentation on my fork, so you can see it live before approving changes.

Preview the working build: https://github.com/Ab2nour/forest-confidence-interval/actions/runs/14481813055

Preview the updated documentation: https://ab2nour.github.io/forest-confidence-interval/

Let me know if there’s anything you'd like me to adjust!