Badge Request: Support ReadTheDocs Business Tiers

:clipboard: **Description**
Currently the support for ReadTheDocs only consist of their free community tier. However, we wanted to use shields to display badges for projects hosted on their Business tiers as well.

:link: **Data**
The data can be fetched very similar to how it is being done currently on the [readthedocs service](https://github.com/badges/shields/blob/9daae0ce94eb8a8ab274faa364c5c21761ce0035/services/readthedocs/readthedocs.service.js#L49), the main difference here is adding support for `readthedocs.com` as well.

ReadTheDocs business do support private repositories, getting badges for those may require a token which would be provided by the user.

:microphone: **Motivation**
Use shields with projects hosted on readthedocs.com . Usage of dynamic URLs are not as feasible since the color would be static. And we do not want to standup our own endpoint to get the badge. Supporting this would be a good addition to shields.

Start with services/readthedocs/readthedocs.service.js at the linked data-fetching code and compare the Read the Docs Business API requirements, including private-repository token handling. Confirm the supported badge URL and response cases, then verify that business-tier projects, including any required authentication path, return the intended badge data.

Written by the indexing model from the issue text.