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.
In the default text output mode, list inactive slots.
In CSV output mode, list inactive slots as additional information;
add output line with number of missing slots and a list thereof.
Also document --csv output mode.
