Add Firefox.com (Springfield) specifics

[janbrasna]

Generally similar to Mozilla.org (Bedrock), just places explicitly listing specifics as:

https://mozilla-l10n.github.io/documentation/products/mozilla_org/manage_review_mozilla_org.html#content-flow-between-bedrock-and-www-l10n

are now not described for Firefox.com — whether extending the current page to either–or form covering both, or cloning that to a mostly duplicate second page with distinct paths would work better?

(The same may be true for localizer docs as well: https://mozilla-l10n.github.io/localizer-documentation/products/mozilla_org/testing.html … that also could group both, but may need describing the endpoints separately.)