Added initial algorithm page

@ChrisBeaumont - this is just a draft for now, and I plan to add correctly-sized and named versions of the plots (and rebase to avoid large binary diffs), but I'd be interested in feedback on what's already there. Once we finalize this page, then I can go back to making the screencast and then embed it at the top of the page for anyone who can watch it instead of reading the text.

Preview of the docs: https://dendrograms-astrofrog-v2.readthedocs.org/en/latest/algorithm.html

What do you think? Too many plots? Not clear enough?