Fix copy-paste error: master_doc -> main_doc
The master_doc
variable in doc/conf.py
was accidentally
renamed to main_doc
in #356 (closed). It is a Sphinx configuration
variable and is not related to the name of the main branch.
Instead of reverting it back to master_doc
, rename it to
root_doc
, which is the new name of this variable in Sphinx 4.
Consequently, we also now depend on Sphinx >= 4.