Refresh documentation site

[edwardchalstrey1]

Changes

• Adds developer docs section: Closes [Documentation]: Update docs for installation, contributions #541
• Simplifies the homepage
• Rearranges the order of sections and adds some external links
• Closes [Documentation]: Robust documentation builds #543
• Prevents most GH actions if only changes made are to docs or GH actions

TODO:

• Since Readthedocs currently defaults to "stable" (the most recent release) as oppose to "latest" (master branch) we need to either make a release after these docs changes, or change the behaviour such that "latest" is default This PR can be part of the imminent 16.4 release
• Remove new logo in this PR and just keep old one

Reviewers

I have enabled docs preview builds for PRs on Readthe docs, you can view the build for this one here. The link is also present in the list of GH actions that ran below - for future updates to docs pages, the other GH actions should no longer run after this PR is merged, so it will be easy to find the Readthedocs PR preview link

[edwardchalstrey1]

Hi @tturocy @rahulsavani - I have made some updates to the Gambit documentation and website based on the idea of revamping the project/software's outward appearance as we discussed. This PR has the changes to the documentation, which are minimal in terms of content, whereas the website changes are much more significant.

Could I get you to do the following:

1. In order to review the docs site changes in this PR, it would be great if we could set up previews via Readthedocs
   • I'll need one of you to add me there so I can do that.
   • I can probably also configure this to auto-build on push to master branch, because it looks like that didn't happen after my previous PR was merged
2. Take a look at the logo design in [Documentation]: Create a modern logo for the Gambit documentation and website #544 and let me know your thoughts - It's included in this PR but can absolutely be switched out for something different!
3. For reviewing the website changes, the easiest thing will probably look at this together next week as it's still a WIP in terms of content.
   • If you want, you can build the site locally to see the design I've gone with in advance.
   • Could I be given full admin access to the gambitproject.github.io repo? I was hoping to set up a PR preview for this so you could just click a link, see my notes here: Generate link with how changes to the site look on PR gambitproject.github.io#14

[edwardchalstrey1]

With the old logo restored for the time being these changes should be good to merge and include in the upcoming release