[OSPO Book] Add InnerSource as an internal area where OSPOs engage

[alice-sowerby]

From #603 from @Pizza-Ria

How an OSPO Works Inside an Organization
As part of either sideways or maybe even looking internally

include efforts vis-a-vis InnerSourcing.

Agreed, a mention of innersourcing here would be appropriate.

[fer-correa]

This is especially important because it allows us to have (in the near future) some link to the information in the innersource chapter that is under development.