Add warning to docs fetched by docs.nextstrain.org

[eharkins]

Description of proposed changes

Background / context:

• Fetch docs instead of using submodules docs.nextstrain.org#36
• https://github.com/nextstrain/docs.nextstrain.org#fetching-of-documents-from-other-repositories

We fetch some docs from this repository into docs.nextstrain.org (see above) to include them in the build for that repository (ie the main docs build). This adds warnings to the first few lines of every such file in this repository, so that if one were to open such a file in docs.nextstrain.org and try to edit it there as if it belonged in that repository, one would hopefully be reminded where the file actually came from and edit it there (here in this repository). It seems it would otherwise be easy to make this mistake and so this warning seems warranted and hopefully will resolve confusion until we have a different, hopefully more intuitive, implementation - namely nextstrain/docs.nextstrain.org#27.

If we don't like having these warnings permanently added to documents in this repository, we could always inject the warning into files once they are fetched into docs.nextstrain.org.

Testing

Local build works and warnings do not appear in the built version of docs.nextstrain.org