This brings the repmgr documentation build system in line with that
used by the main PostgreSQL project, and removed the restriction
that documentation must be built against PostgreSQL 9.6 or earlier.
Main formatting changes are:
- convert empty-element tags (mainly <xref/>)
- put <indexterm> sections in the correct location
- correct usage of various entities.
The configuration file path is stored to make remote execution of repmgr
(e.g. during "repmgr standby switchover") simpler, so relative paths
make no sense.
Addresses GitHub #332
