Document how to expose block plugin schemas on BlockPluginInterface

Problem/Motivation

Block plugins' configuration is saved as part of block entities. As such the configuration of every block plugin needs a schema, but nothing on this topic is documented on BlockPluginInterface.

Proposed resolution

Document how to add a block plugin schema on BlockPluginInterface.

Remaining tasks

None.

User interface changes

None.

API changes

None.

Data model changes

None.

Beta phase evaluation