Update checks documentation

Updates documentation to reflect recent changes to checks, primarily for the addition of a new check type branches_exist.

Depends on lhcb-dpa/analysis-productions/lbaplocal!30 (merged) and lhcb-dpa/analysis-productions/lbapcommon!38 (merged)

Work related to https://gitlab.cern.ch/lhcb-dpa/project/-/issues/93