Deduplicate clades pages

[eharkins]

This work began as nextstrain/nextstrain.org#170, but post-migration will need another look here. The idea was merging / reducing any duplication between:

1. https://nextstrain.org/docs/tutorials/defining-clades (moved to https://docs.nextstrain.org/en/latest/guides/bioinformatics/defining-clades.html) and
2. https://docs.nextstrain.org/projects/augur/en/latest/faq/clades.html (unchanged in augur subproject after migration)

[eharkins]

@jameshadfield would be good to know if you still feel similarly on nextstrain/nextstrain.org#170 (review) given nextstrain/nextstrain.org#170 (comment) and the new context.