Instructions for installing the flatsurf-stack, a logo and some documentation cleanup

[saraedum]

apologies for the logo, graphic design is my passion™, but likely at some point @alejandrarobles is going to create a nice logo for us.

Fixes #110, #161

TODO

• [x] List Related Projects on the flatsurf.github.io page instead. (https://github.com/flatsurf/flatsurf.github.io/pull/3)
• [x] Mention this project prominently on the flaturf repository README (maybe rename it to libflatsurf.)
• [x] Add @wphooper to .zenodo.json. (What's your ORCId?)
• [x] Change logo URL in README.
• [x] Explain how to install the flatsurf stack with mamba.
• [x] Explain how to upgrade the flatsurf stack with mamba.
• [x] Add contact section to flatsurf.github.io. (https://github.com/flatsurf/flatsurf.github.io/pull/3)
• [x] Link to contact section from here.
• [x] Explain how to install inside a SageMath source build.
• [x] Explain how to pip install without optional dependencies.
• [x] Explain how to build from source with conda dependencies.
• [x] Explain how to build from source in a SageMath source build. (maybe we don't want to teach people how to do that actually.)
• [x] Explain how to build from source with pip.
• [x] Explain how to uninstall in all of the above.
• [x] Link to the installation instructions from flatsurf.github.io and github.com/flatsurf.

[saraedum]

@videlec @wphooper should we keep "A tour of sage-flatsurf"? It's "under construction" and it seems to me that the "Defining Surfaces" example is probably more useful to most.

I could maybe just rename the entire Examples section to "A Tour of sage-flatsurf". What do you think?

[videlec]

@videlec @wphooper should we keep "A tour of sage-flatsurf"? It's "under construction" and it seems to me that the "Defining Surfaces" example is probably more useful to most.

Definitely.

I could maybe just rename the entire Examples section to "A Tour of sage-flatsurf". What do you think?

Sounds good to me.