Documentation

[barendgehrels]

I would like to see the following changes:

• the release documentation should call b2 (if it does not yet do that)
• make_qbk.py should not call doxygen but it should be called before
• the index/Doxyfile should be merged into the main one
• the index/make_qbk.py should be merged into the main one

Such that we have one main doc, not two, and a clear workflow with clear individual steps
Are there any objections to this? @awulkiew , agree from index perspective?