CompatHelper: add new compat entry for AbstractMCMC at version 5 for package docs, (keep existing compat) (#1114)

Co-authored-by: CompatHelper Julia <compathelper_noreply@julialang.org>

Author: github-actions[bot]

docs/Project.toml

@@ -15,6 +15,7 @@ MarginalLogDensities = "f0c3360a-fb8d-11e9-1194-5521fd7ee392"
 StableRNGs = "860ef19b-820b-49d6-a774-d7a799459cd3"
 
 [compat]
+AbstractMCMC = "5"
 AbstractPPL = "0.13"
 Accessors = "0.1"
 Distributions = "0.25"