Add check documentation page for 2 concept plausibility checks

[katy-sadowski]

plausibleGenderUseDescendants and plausibleUnitConceptIds need check-level docs pages. A template for plausibleUnitConceptIds is already available here: https://github.com/OHDSI/DataQualityDashboard/blob/main/vignettes/checks/plausibleUnitConceptIds.Rmd. The page for plausibleGender can be renamed/repurposed for the new version of this check (https://github.com/OHDSI/DataQualityDashboard/blob/main/vignettes/checks/plausibleGender.Rmd).