Migration assistance for operators

[arbrandes]

Description

In order to help ease the migration to frontend-base, investigate ways that existing env.config.jsx files can be re-used, then select and implement the ones that are judged to be sufficient. This can take the form of:

• Specific documentation on how to convert one or more env.config.jsx files into a single site.config.tsx one

[arbrandes]

Closed in favor of #222 and #258.