Docs: Fly.io tutorial

[paulrobertlloyd]

Fly.io is a popular hosting platform, and users have experienced problems setting up Indiekit servers using it, see #588.

• Add tutorial for setting up an Indiekit server on Fly.io (and connecting to a MongoDB database)
• Ensure tutorial mentions how to save environment variables, as this is a common stumbling block, see:
  • Cannot authenticate into Indiekit (hosted on Fly.io) #588 (comment)
  • Cannot authenticate into Indiekit (hosted on Fly.io) #588 (comment)