Set up GitHub Actions for builds

[v--]

I assume this is related to #4. Like I promised, I set up a GitHub Action for building PDFs so that they do not have to be included in the repository. I know that there are a lot of moving bits but I tried to do it without much assumptions. Here are some highlights:

• The entire process relies on GNU Make, hence changing any of the moving bits of the build process should amount to changing the makefile. I'm not the biggest fan of GNU Make but it is the de-facto standard for builds.
• I removed the PDF from the repository. Instead, it is built automatically on pushes to the main branch and is then deployed to a separate branch via GitHub actions. This branch is tied to GitHub Pages so you get a project page like this one I set up.
• I made a basic HTML file with some styles that could also serve as a landing page. This is currently required because the site is deployed along with the PDF.
• I modified the main tex file to include a "build revision". The purpose of this is to know when the PDF has been built so that if you download two distinct versions of the PDF you get different revisions and you know that they are different.
• I added a packages directory for the single macro I introduced. I think it is a starting point for extracting some of the macros outside of the tex files.

Of course, this can be easily extended to other formats by changing only the makefile and the actions file.

I haven't really discussed this with any of you so feel free to comment/request changes on any of this.

[v--]

@abackus In order for the project to have a landing page like the one linked in the pull-request, somebody (I guess you) should enable GitHub Pages in the project setting and point it to the automated_website_build branch. The URL should then be something like https://clopen-analysis.github.io/latex.

[qwinters]

thanks @v-- for setting this up!!