Draft: docs: Add development environment setup under WSL.

This is a work-in-progress MR documenting how to create a Pitivi development environment by running Ubuntu 20.04 or a similar distro under Windows Subsystem for Linux (WSL).

Questions on the process itself:

• I'm betting that Step 11 could be better since the OS's libgstpulseaudio.so might be old. Are there options for building a .so providing the pulsesink element in the gst-build environment? Similar questions apply to the other three plugins that some students needed symlinked to get the test cases passing.

• It seems surprising that Step 12 is necessary, especially since the data/pixmaps is added to the Yaru icon theme's search path when I enter the gst-build environment. Did I overlook something earlier?

• In Step 13, I am copying and patching the pre-commit hook instead of symlinking it. Obviously it would be better to submit an MR to make the hook work under WSL by default, but I wanted to get your opinions and first make sure that looking for pre-commit with which is safe under all environments. The blame suggests that care might be required with GNOME Builder, which I have not used. If the hook can't be symlinked, then the first paragraph in …/docs/Precommit_hooks.md should probably acknowledge that that the hook could actually be a copy.

Questions on the documentation:

• Should crossplatform.md link to WSL.md?

• If so, should WSL.md be under HACKING.md or crossplatform.md in sitemap.txt?