Redirect /docs and /help to docs.nextstrain.org

[eharkins]

Description of proposed changes

As per nextstrain/docs.nextstrain.org#13, we need to send what used to be nextstrain.org/docs and nextstrain.org/help both over to docs.nextstrain.org since we are migrating our documentation there. This does that in three steps:

• replace all the links (e.g. in the navigation bar at the top of nextstrain.org) which previously when to these pages to external links that send you to docs.nextstrain.org
• redirect all current nextstrain.org/docs/* and nextstrain.org/help/* urls to the corresponding pages at docs.nextstrain.org
• remove the (mostly markdown) content that we have migrated to docs.nextstrain.org from this repository, so that Gatsby no longer builds it into the static site.

Related issue(s)

nextstrain/docs.nextstrain.org#13
nextstrain/docs.nextstrain.org#35 (in case we want to adjust redirects for clade labeling page)

Testing

I've just been testing this locally with gatsby (npm run dev in static-site and npm run server from the root directory), checking that redirects work and clicking on external links to docs.nextstrain.org. I will deploy this on heroku to test today.

[eharkins]

It's necessary to have at least one instance of anchorText or else gatsby will complain.

[eharkins]

Upon or before merging, we'll want to remove the banner from the docs.nextstrain.org home page (https://github.com/nextstrain/docs.nextstrain.org/blob/master/src/index.rst) that says:

This site is currently only a stub, to show what's possible with Read The Docs for an umbrella documentation project.
For the real documentation entry point, please go to nextstrain.org/docs.

Since going to nextstrain.org/docs will redirect back to docs.nextstrain.org.

[jameshadfield]

Looks good @eharkins - have left specific line-comments for minor changes. Thanks!

[eharkins]

@jameshadfield 4d30a24 should resolve all of the above, let me know if it's not the case, thanks for the review!

[eharkins]

Merging nextstrain/docs.nextstrain.org#36 would / will mean some updates are needed here. Since there were only a few changes I just added them in comment suggestions which we can add, instead of creating yet another PR into this one.