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.
From the next major release, the current documentation will be located in the
"/docs/current/" subdirectory. This makes it easier to provide canonical
links to the latest version of the documentation (similar to how the
main PostgreSQL documentation is organised).
Once the following major release is available, the documentation will be moved
to a subdirectory with the version number, e.g. "/docs/4.3/".
We need to emphasise that the repmgr packages are only compatible
with packages based on the PGDG filesystem layout; 3rd party vendor
packages often put application and data directories elsewhere.
See e.g. GitHub #427.
