Document feature flags

[FelixMalfait]

We recently shipped feature flags but did not document it

We should create a new page within the "Backend Development > Others" section. In that page we should explain how to enable and disable a feature flag (it's just adding a row in the Postgres database)

The list of available feature flags is currently visible here: https://github.com/twentyhq/twenty/blob/main/packages/twenty-server/src/core/feature-flag/feature-flag.entity.ts#L17

[charlesBochet]

Yes, good point!