Create custom domain for Read the Docs site

## Issue description

Given the recent updates to the website routes (see https://github.com/dandi/dandi.github.io/issues/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](https://docs.readthedocs.io/en/stable/guides/custom-domains.html) 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.