Allow setting maxbatch to adjust chunk size for parallel read/write

Is your feature request related to a problem? Please describe.
I'm currently working on a project with a large documentation (~22k files).
We noticed that reading in parallel is particularly slow:

sphinx-build init        0:00:47.712593
sphinx-build read        0:09:39.427063
sphinx-build write       0:03:55.736320
sphinx-build copy, dump  0:04:10.119199

I managed to track this issue down to the calculation of the chunk size for parallel processing:
https://github.com/sphinx-doc/sphinx/blob/cc314f13e8a98393ab018d83d8957a724a6f338a/sphinx/util/parallel.py#L137-L150
When setting maxbatch from 10 to 1000 the read performance improves significantly:

sphinx-build init        0:00:45.914173
sphinx-build read        0:00:50.814347
sphinx-build write       0:03:36.089072
sphinx-build copy, dump  0:04:06.186373

Describe the solution you'd like
It would be ideal if Sphinx provided a means to set this value manually, e.g. as an argument for sphinx-build.

Start by reading sphinx/util/parallel.py around lines 137-150, then trace the sphinx-build argument and configuration entry points for parallel processing. Done means users can set maxbatch manually and that value controls the parallel read/write chunk size; compare the reported sphinx-build phase timings to confirm the setting is applied.