Bunch of improvements

[ghost]

This PR packs a bunch of improvements to the current webpage. (I've realized the project's main language was in English midway, so, apologies for any commits in Spanish).

These so-called improvements are the following:

1. Grammar / Spelling corrections.
2. Alternate texts for some sections.
3. Image minification.
4. A GitHub Actions Pipeline to build + minify assets.
5. CSS Improvements.
6. Webhint corrections to stay close to the w3c standards.
7. JS improvements.
8. SEO Improvements.

There are some things I left behind in this PR, so I can submit another one or make a proposal for easier maintenance:

1. Not all pages have the webhints corrections, most Desktop Environment pages still lack some corrections, also about.html and wiki.html have no webhint corrections. However ALL pages have the basic corrections such as a valid HTML language, a charset attribute and a meta description.
2. I did not optimize .svg images because I don't know if those are still a WIP or final.
3. I didn't remove all inline styles.

Changes in site score

Original Site

url: https://cereuslinuxproject.github.io/
score url: https://pagespeed.web.dev/report?url=https%3A%2F%2Fcereuslinuxproject.github.io%2F

This PR site

url: https://ventgrey.github.io/CereusLinuxProject.github.io/
score url: https://pagespeed.web.dev/report?url=https%3A%2F%2Fventgrey.github.io%2FCereusLinuxProject.github.io%2F

Proposal for future revisions

If wanted, I could propose migrating a site this big (and with those reusable components) to a static site generator like Hugo. This will make things easier to maintain and even remove the used JS for navbar inclusion completely.

[KF-Art]

Thanks a lot for your contribution and improvements. I have tested them and work well. Now it is merged.

[ghost]

In any case I wish to suggest migrating the site to Hugo. It's a statuc site generator like Jekyll. This would result in easier maintenance for future website revisions.