They were disabled on master as they make builds way too long,
but for the stable branch they make sense, as it is synced manually.
(cherry picked from commit 763f6a9555)
Reference: https://docs.readthedocs.io/en/latest/yaml-config.html
`htmlzip` target is too heavy and slows down build a lot (or makes it timeout),
so disabling for now. We could reenable it if an option is given for a non
`singlehtml` target, see https://github.com/rtfd/readthedocs.org/issues/4190.
Also disabling `epub` target for now, as it also makes builds slower and we
iterate too fast on the master branch for it to be meaningful.
We could eventually build ePub versions manually once a month or so and upload
those somewhere if there's demand for it.
(cherry picked from commit ae7277c1e4)
Rémi Verschelde