Add a reference from Bolt Extensions documentation to Bolt Internals to minimise confusion for accessing Container elements

Currently there's a missing link between Extension docs and Container documentation, as quite often you want access to the container, but until you really start digging throughout other bits of the docs until you stumble upon Bolt Internals.

Locate the Extension documentation and the Bolt Internals or Container documentation referenced in the issue. Add a clear reference from the Extension documentation to the Container information, then verify that readers can find how to access Container elements from the Extension docs.

Written by the indexing model from the issue text.