doc/source/projectconf.rst: Documenting plugin loading section.

This fixes issue #183.

Also move the `format-version` related documentation to the first
section "Essentials", beside the project name and element path, since
this is a quite global option it belongs here and not hidden away
with the plugin loading documentation.

Also adjust the main index.rst to include the plugins sub-section
as an adjacent sub-point of the project configuration (consistent
with other project configuration sections).