improve contributor tech guide for Windows, and also for non-techies

As [has been discussed](https://forum.opendatakit.org/t/feedback-from-contributor-who-tried-to-help-with-docs/14606), the docs guide doesn't work well for Windows or for people without a lot of command line experience.

I have recently sat with someone brand-new to ALL of this (open source, git, cli, etc) working on a Windows machine, and walked through all the processes, taking notes. I am working on a rewrite of this guide based on that.

#824 is a requirement for completing this. So I'm going to attempt it.

Start with the existing contributor tech guide and the notes from the Windows walkthrough described in the issue. Review #824 first because it is required, then revise the guide so Windows users and people new to open source, Git, and the command line can follow the documented process from start to finish.

Written by the indexing model from the issue text.