Rewrite BobaEditor getting started page

[cyrusae]

This is part of #13 , but BobaEditor is currently functioning as the test for what those rewrites should look like, so it gets its own issue because that's a different task than putting them into practice and I find minutiae-flavored updates motivating.

• Change clone instructions to "clone and fork" instructions
• Remove use of "install" to refer to checking out the code
• Add "Test Your Changes" section
• Add small "and then submit a pull request" nudge
• Make list of dependent/related documentation this makes want created or updated

[cyrusae]

A note on my documentation logic: I lean towards more explicitly trying to ease in people new to this kind of thing. This produces "fluff" compared to a standard readme style. This is deliberate. Concrit on the calls to action/inline explanations/etc. is welcome but they're there for a reason; the reason is that BobaBoard's intended audience is disproportionately likely, as I think anyone reading this is going to have experienced the mortifying ordeal of knowing (yours truly included), to self-select out of getting involved.