Input checker documentation missing

[bhargavasana]

Input checker documentation was added in #753 (https://github.com/ActivitySim/activitysim/pull/753/files#diff-d606c1bcd72689c8bb949dcdafe4bcd0420da5cbaa7daf9614f0757734cffaf7) but this appears to be missing in the current documentation website https://activitysim.github.io/activitysim/v1.3.1/index.html

[dhensle]

Fixed by #941