Migrate RDT configs

[pombredanne]

From RTD email:

Hello,
The Read the Docs build system will start requiring a configuration file v2 (.readthedocs.yaml) starting on September 25, 2023. We are sending weekly notifications about this issue to all impacted users, as well as temporary build failures (brownouts) as the date approaches for those who haven't migrated their projects. The timeline for this deprecation is as follows:
Monday, July 24, 2023: Do the first brownout (temporarily enforce this deprecation) for 12 hours: 00:01 PST to 11:59 PST (noon)
Monday, August 14, 2023: Do a second brownout (temporarily enforce this deprecation) for 24 hours: 00:01 PST to 23:59 PST (midnight)
Monday, September 4, 2023: Do a third and final brownout (temporarily enforce this deprecation) for 48 hours: 00:01 PST to September 5, 2023 23:59 PST (midnight)
Monday, September 25, 2023: Fully remove support for building documentation without configuration file v2.
We have identified that the following projects which you maintain, and were built in the last year, are impacted by this deprecation:
intbitset
Please add a configuration file to your projects to ensure that they continue building successfully and to stop receiving these notifications. If you want to opt-out from these emails, you can edit your preferences in your account settings. For more information on how to create a required configuration file, read our blog post Get in touch with us via our support and let us know if you are unable to use a configuration file for any reason.
Keep documenting,
Read the Docs