Update README link locations

[timothy-nunn]

README links still point at our GitLab pages.

[rkemb]

Docker and Singularity installation instruction links point to git.ccfe.ac.uk

[jmorris-uk]

Other minor docs fixes that might as well live here:

1. Vardes page width needs to be wider -- table renders awkwardly
2. Change "Install on Ubuntu/WSL" to "Install on Ubuntu/Windows" and then detail WSL on Windows in the body of the page.
3. Rename "Home" to "Getting started"
4. Need to update CI page with high level table of what each GitHub action is doing.

[timothy-nunn]

@jmorris-uk I think mkdocs makes the massive margins, so I will have a little look into that. For now, I have updated the table styling to make it a bit easier to use.