Yes, they do "stack"
In Chapter 6 of the PHB, under "Spellcasting" (p. 164), the following paragraph can be found (with additions by me in brackets):
Pact Magic. If you have both the Spellcasting class feature [e.g. via cleric] and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the Spellcasting class feature, and you can use the spell slots from the Spellcasting class feature to cast warlock spells you know.
Since the Spellcasting and Pact Magic class features are separate class features, although they both give you spell slots, they are still different class features, so the spell slots "stack" because they come from different sources.
Of course, when you short rest, you only get the slots back from your Pact Magic, you must complete a long rest to get all of them back (i.e. those from a cleric's Spellcasting).
Cantrips also stack
Yes, cantrips stack too. You get 2 cantrips via warlock and 3 cantrips via cleric; these cantrips must come from their respective spell lists, but otherwise, having cantrips from one source does not exclude you from gaining more cantrips from another source (so long as you stick to the spell list of the class granting you the cantrips at that time).