Create custom domain for Read the Docs site

[kabilar]

Issue description

Given the recent updates to the website routes (see dandi/dandi-about#84) and particularly the renaming of the DANDI Handbook to docs.dandiarchive.org, as @yarikoptic points out it is confusing that we have a dandi RTD site for the DANDI Client.

Possible solution

I propose that we create a custom domain for the RTD site and use docs.dandiarchive.org/api/client or docs.dandiarchive.org/client. RTD allows for custom domains but we will need to investigate further if we can use a custom domain that also includes a path (e.g. /api/client).

If we agree on this being the solution, I would be happy to work on this. I will just need the credentials for the dandi RTD account.