Explain feature flags in Frontend Docs

[CharlotteDowns]

What

We decided to add a new page within the Frontend docs to explain feature flags.

This new page will include

• what feature flags are
• why we use feature flags

Plus table/consistently formatted content of active feature flags, with fields to include:

• variable name
• purpose
• version (or date) introduced
• expected retirement

Why

We don't think this information is easy to understand for users. Information we have is distributed across places.

Who needs to work on this

@seaemsi

Who needs to review this

@calvin-lau-sig7, rest of squad

Done when

• A list of optional features/things behind a feature flag has been collated
• We have a way of categorising the list
• There is a general introduction to what optional features are and why we use them
• There is information on the rebrand specific optional features
• [Stretch] There is information on the entire list of optional features

[calvin-lau-sig7]

This will be a new page, tbc whether we call them feature flags.

Intro will explain:

• what a feature flag is
• why – we sometimes release new features, but want to give teams time to adopt them
• so instead of making a breaking change, we provide the option to activate them, in the form of a feature flag

Table:

• variable name
• description: including where it's structured
• purpose
• version (or date) introduced
• expected retirement

[domoscargin]

@seaemsi I'm moving this to small stories, as I think we've done most of the work here. Might be worth updating the description/leaving a comment with the current state of things.