docs: set language to english
It seems recent versions of Sphinx has started issuing a warning when language isn't set to a recognized value. Since we're building with warnings as errors on CI, this causes issues for people.
We're writing English docs, so we should set this to English. I probably confused this setting with the highlight_language setting or something when I set this up initially.
Fixes: 00cd1346 ("docs: use sphinx") Closes: #6711 (closed)
Edited by Erik Faye-Lund