Update docs for language-generic audience

[anna-stacey]

• i.e. docs focussed on users who want to make their own FST, not just build the Ojibwe one
• in particular, need something written about the tests, and how to use them in general, not just for Ojibwe

[anna-stacey]

Thanks to recent commits overhauling docs/ this is almost done. Lingering things:

• preverb details are too Ojibwe-specific, and should be rewritten to use that more as an example
• still need docs on adding support for a new lang/dialect (but there is already an issue about this)