Deprecate 'build_rst_docs' in favor of 'build_base_docs'

PySide supports rst and markdown files. The old command name is therefore not reflecting this and misleading.

1. Deprecate the command in favor of 'build_base_docs'
2. Remove the command in the next release

Following files would need an update:

build_scripts/main.py, in PysideBaseDocs
build_scripts/options.py
build_scripts/setup_runner.py