Pulp

Docs need to be added to convey several points that are new with 3.0.

- Starting with Pulp 3.0 the plugin API which will be versioned differently from platform.
- The plugin API with 3.0 will be governed by semantic versioning
- With 3.0 it will be launched as 0.1 which allows us flexibility to make backwards incompatible changes until the plugin API reaches 1.0
- The docs need to state what the Plugin API is for a given version of Pulp. For example, in 3.0 docs they will state the plugin API is at version 0.1.

A release note on the plugin API should be added which links to the plugin API docs. These points ^ should be near the top of the Plugin API docs.

This assumes that #2179 is already done and adds docs to it specific for 3.0. These should land on 3.0 since they are unique to it.