Add .md document for controller upgrade to `anoncreds`

[jamshale]

Using a controller to upgrade an agent or tenant to anoncreds needs specific instructions for the effected endpoints, payload changes and triggering the upgrade. This should be done in a way that makes it as easy as possible to implement and consider backwards compatibility. For example in the case an upgrade fails the controller would still support an askar wallet and if the the upgrade succeeds, it will use the new anoncreds api. The old endpoints can be removed in a controller version after the upgrade is confirmed.

This is directly related to the pending PR and linked issue found at https://github.com/hyperledger/aries-cloudagent-python/pull/2840