Update extensions migration documentation

[peternied]

After the following work is done, make sure it has been documented in the extensions migration guide so that an extension can use the new permission to read from existing system indexes from its previous incarnation as a plugin.

• [Feature] Update SecurityIndexAccessEvaluator to support new index permission #2553

[peternied]

@DarshitChanpura I think there is value to this work so far as when more time is invested in extensions it should be clear how to transition from the super user permissions kind of ecosystem to using the system indices permissions. However; this work could be done sooner or later before extensions are generally available. Maybe this issue could be attached to another documentation meta issue / tracking issue.

[DarshitChanpura]

Let's track it in a separate meta for documentation for everything related to extensions. I've added this to: #3354