Documentation of environment variables and storage of secrets

[isedwards]

Currently developers are working with a shared copy of environment variables - some of which contain secrets.

• [ ] Non-secrets should in included in the installation documentation (https://github.com/opencdms/surface/issues/83)
• [ ] Secrets for surface.opencdms.org deployment should be added to the GitHub secrets used for deployment
• [ ] Secrets for NMS of Belize should be limited to Eligence and NMSB only

See also #21