docs(pro): Asset Identity is a Feature Flags toggle, with how to turn it off

[blakeaowens]

Description

Documents the Pro Asset Identity feature flag, which replaces the DD_V3_ASSET_ALIASES deployment setting as the switch for per-source asset aliases.

• Feature Flags reference: a new Asset Identity row (off by default).
• Asset page, "Turning Asset Identity on and off": what the flag changes, that turning it on changes nothing about Records already mapped (each Connector records its identifier whenever a Mapping is created or changed, whether the flag is on or not), that a change takes effect without a restart, how to turn it off again (the toggle, or python manage.py set_feature asset_aliases False), and what happens to an instance that had DD_V3_ASSET_ALIASES=True.
• Managing Records, Auto-Mapping: steps 1 and 2 now point at the Asset Identity flag instead of the environment variable, in both the Asset and Product wording.
• The backfill_oci_aliases paragraph refers to the flag by its new name.

English pages only; the language copies are auto-translated. This ships with the matching Pro change in 3.4.300.