Solo: Improve documentation

[conorpp]

Documentation can always be improved, at least need the following:

• [x] external site set up
• [ ] tutorial on writing extension / example
• [ ] document how to program board #4
• [ ] document key derivation
• [ ] porting guide
• [ ] How to verify firmware is legit

[nickray]

Documentation is now built from the contents of this repo's docs/, configured via mkdocs.yml, and served at https://solo.solokeys.io/

[corbolais]

hey, great job!
now it's gotta be filled w/ some more content..
nice domain name, btw :)

[nonnymoose]

Could you also add notes on how firmware.json works in the docs about the signed update process?