docs: simplify outdated dev instructions

Summary:

Our dev instructions here are quite old and contain either out of date or duplicate information that is already in dev scripts. So here I removed some unnecessary and out of date details and pointed to the dev scripts instead. This way we don't have to keep updating this instructions along with those files. I did my best to keep the useful examples in the "Manual Tests" section around, such as loading test data into the database and querying the local API directly from the command line (though the diffs make it look like I deleted all that, it's actually kept around unaltered below).

Prerequisites:

[x] Unless it is a documentation hotfix it should be merged against the dev branch
[x] Branch is up-to-date with the branch to be merged with, i.e. dev
[x] Build is successful
[x] Code is cleaned up and formatted

cmu-delphi / delphi-epidata

docs: simplify outdated dev instructions #1466

Summary:

Prerequisites:

Quality Gate passed