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.
If "archive_cleanup_command" is defined in "repmgr.conf", a corresponding
entry will be made in the node's "recovery.conf" file after cloning a
standby.
Note that we recommend using PgBarman to manage WAL archives, but are
providing this facility to help repmgr to be integrated in existing environments.
Implements GitHub #416.
