Add Windows instructions to contributing page

[GeckoEidechse]

In this page: https://r2northstar.readthedocs.io/en/latest/guides/contributing.html

source venv/bin/activate does not work on Windows.

Neither does make html.

[GeckoEidechse]

The Windows equivalent of source venv/bin/activate is venv\Scripts\activate.bat

make html expands to sphinx-build -M html source build. It probably makes most sense to use that with Windows users ^^