Remove core from combined documentation.

They want it to be standalone.

This reverts:
- commit 88303ffaa15c293c14e207b7e62918210f471b03.
- commit 84779968902e151f6c1e17cd91e063e35f701d24.
- commit 52ea2044ced3b2be744e5036be953f7b133ca873.

Conflicts:
	doc/Jamfile.v2
	doc/src/boost.xml