Retire the "Setting up Your Development Environment" Google doc

[seanh]

Problem

The documentation for how to set up a Hypothesis development environment from scratch currently lives in the Google doc Setting up Your Development Environment. Nobody likes Google docs, the formatting is a pain, and Hypothesis seems to be moving away from Google docs generally.

Solution

1. The plan is to tidy up and make consistent the dev env setup docs in the project's themselves which will make the Google doc much shorter as it'll only need to link to the HACKING.md files of the projects the user should set up, it won't need to repeat the install steps itself. See Deduplicate development environment set up docs, and make them consistent #14

2. What's left of the Google doc is then planned to move into the onboarding repo.

3. We should then retire the original Google docs: perhaps tombstone is by replacing the contents with a link to the onboarding repo's copy, perhaps move the doc to an archive folder, perhaps delete it

[seanh]

Closing this in favour of #22, #14, and #17