Add background and about docs

[choldgraf]

This PR adds a bit of background information to this project, and updates the docs a little bit in general. The goal is to provide high-level motivation and context for our goals and why the project exists. Here are the main additions:

• Added an "About FRX" page that describes the key ideas behind the paper and where this project tries to fit in. This is heavily inspired by the discussion @yuvipanda led in Pick a name for this open source project #15
• Added some ideas for the functionality and goals of the project at a high level
• Updated README.md so that we re-use it as the landing page for the website as well
• Some minor website updates

I decided to write this up as a way of helping me learn more about the motivation behind the project in general, so hopefully these ideas are correct and helpful to others!

This likely feeds into the following issues:

• [EPIC] Launch the frx-challenges open source application #153
• And much of the frx.md copy could be re-used for Write a blog post describing the launch of this open source project #156

[choldgraf]

I made a few updates to your comments and committed - lemme know if that looks OK! And feel free to merge if you or others want to.

[yuvipanda]

ty @choldgraf

[choldgraf]

Is there a reason not to talk about this project? Eg just throwing excitement towards it on social media or informal conversation? I know we are planning to "launch" it formally but I'm a fan of communication throughout the process rather than waiting until the end. What do others think?

Note that we absolutely still would need to write up the big post etc, I am just talking about informal comms leading up to it

[yuvipanda]

@choldgraf mostly that there is no real documentation, so if people come see there isn't much to be seen. Not a secret or anything like that though.