Automated spell checking

[ehuss]

I think it would be helpful to have some kind of tooling to help check or enforce spelling in the reference. I think a challenge is picking the right tooling for that. Some of the considerations:

• How easy is the tooling to use?
  • How easy is it to install and run locally?
  • How easy is it to add unknown words?
  • How much of a roadblock will this be for contributors?
  • How stable is it?
  • How well does it handle code?
• What dictionary to use?

I'm aware of a few tools, though I'm sure there are others:

• typos
• aspell (used by the Rust Book)
• ispell
• hunspell
• cargo spellcheck (not sure if that supports markdown)
• spellcheck-cli

This is split off from #599.