[PATCH] Updating documentation about bulding documentation in Windows.

Hello everyone.

Current documentation contains outdated instruction and scripts relative to
building documentation on Windows machine.

I removed useless scripts and wrote some guide to build docs using Windows
Linux Subsystem.

Also, could be viewed on github -
https://github.com/michail-nikolaev/postgres/commit/c6781351306810bfebab1fa5b454e8d24b9ea2f6

Thanks,
Michail.