Refactor I/O code; only write/copy files if the destination's content would change (assuming it is not too large)

[felixfontein]

The optimization ensures that if the Ansible docsite build is run twice in a row, Sphinx should be a lot faster on the second run since the input files do not change. (Or only some of them.)

I set the check limit to 256 KiB, which most RST files should be smaller than.

[briantist]

Faster iteration and reduced run times, can't argue with that!

[felixfontein]

This implements a similar logic to the one in ansible/ansible#45062 (except that you can define a maximum size for when to do this comparison).

[felixfontein]

@briantist thanks for reviewing this!