fix formatting and link

closes: https://github.com/rsyslog/rsyslog-doc/issues/87
Merge branch 'v5-stable' into v7-stable
fix doc main page

closes: https://github.com/rsyslog/rsyslog-doc/issues/50
Merge branch 'v5-stable' into v7-stable
add description of input() object
fix interlinking problems
Merge branch 'v7-stable' into v8-stable
forward compatibility: set some reference labels
Merge branch 'v7-stable' into v8-stable
fix build bug