Document how to set up a sandbox server

[Sheeri]

Setting up a sandbox server requires special knowledge, and should be documented to overcome institutional knowledge barriers.

Describe the solution you'd like
Documentation so a reasonably techie administrator can set up a sandbox instance.

Notes:
In the web app, the 'sandbox: true' flag needs to be set in the config.js file so the new_dev_user form is activated.
In the realm app, add developerMode value (under Realm -> Values & Secrets)
In the realm app, under Rules -> mongodb-atlas -> Agency Admin role, edit the role and check "Insert Documents".