This commits deletes all references and code for back-bdb and back-hdb.
There is some follow up work still necessary to flush out the admin
guide for back-mdb.

'Config' is no longer useless since back-config introduction; do not document 'Cache' as it is no longer in use

the --enable-monitor steps are not needed.  Best just to
note this section assumes a default build and briefly note
requirements if built as a module.
Note that some example LDIFs don't include userApplications
attributes.

between documents.  Add -2topics navigation.