GitHub readme remake

[TorkelE]

Updates the GitHub readme page. Also updated the docs/Project.toml file for Catalyst v14.

[TorkelE]

I think the reason that there is lots of diff is that there has been quite widespread rewriting and reordering to most parts of the text, i.e. the changes probably are more widespread than you first thought.

But yes, let's first sort out the spatial SSA PR before we look at this.

[TorkelE]

Ok, this one is read. Two additional comments:

• The (@ref ref) mark where I want to put links to places in the docs. However, I cannot actually put these links in until the rest of the docs have been finished (and released, i.e. I do not want to link dev docs). They are there now so you can see where I want to link stuff. before we merge I will save this file somewhere on my computer (so I can go and see where I wanted to put links), and then remove all of these. And yes, you are totally right that I have to use normal links and cannot use Documenter references.
• Ok, I think there was something else, but I have now forgotten. maybe it was one of the things I wrote as comments to the code though.