The `only::` and `ifconfig::` blocks (any blocks, actually)
are explicitly not intended for altering
doctree structure. Putting structural details in
blocks can cause unexpected and erroneous behavior
across sphinx versions or themes, as it breaks the
underlying document builder API.

This change uses builder-specific configuration elements
to direct (2) different root doctree entry points, where appropriate.
To avoid extra processing (and warnings about orphaned docs),
the unused root document is excluded from processing
in builds where it is not used.

Similarly, unused configuration entries are excluded from processing in
builds for which they are not needed. E.g. we do not define the
`latex_documents` when we are not building them so that we do not get
errors about references to the latex root document (texindex.rst),
which we have excluded from processing.

We use globbing in `toctree` directives to allow exclusion of paths
that are not considered due to `exclude_patterns`.