Add documentation on running flextesa in a dev workflow

[jevonearth]

Make documentation explaining how to get a local flextesa node set up, and how to use it in a development workflow.

• [ ] Internal notion page
• [ ] Generic user documentation for the website, i.e. environment setup

Related to: #457 #276

[roxaneletourneau]

https://tezos.gitlab.io/flextesa/

[hui-an-yang]

https://github.com/ecadlabs/taquito/tree/master/integration-tests#taquito-integration-tests-with-flextesa @michaelkernaghan has documented well how to run integration tests with flextesa. Also, flextesa is no longer maintained; the latest docker image was 6m ago for oxford1. flextesa tests will not be relevant anymore; we have ticket2851 to update our ci script. We will add updating the README.md in integration-test folder, too.