Stop fetching docs from subprojects

[victorlin]

This is a temporary solution as noted in the README.

Possible solutions

• Move fetched docs into this project (Move filtering/subsampling docs into this project #198 (comment))
• Don't include any of the fetched docs in this project's sidebar. Instead, use them in the subprojects sidebar.
• Use a shared TOC across all RTD projects (#27)
  • This is linked to as the solution in the README, but as @tsibley noted, it's somewhat independent and can be done regardless of docs fetching.

Related issues/PRs

• Add warning to docs fetched by docs.nextstrain.org augur#634