#Movie appearances
There are three primary times an AT-AT appears in movies.
##Hoth
The most well known appearance is the Battle of Hoth in Empire Strikes Back, where multiple AT-ATs (trivia: that they are never called this in the films, only "imperial walkers") land on Hoth and assault the Rebel Base.
From viewing the movies, there are many sweeping images of the invasion:
Note that in none of them do you see supporting craft or anything which might allow a "ladder" to get troops down/up. The sole exception is a small scout walker, visible for a short glimpse:
This suggests one of two things. Either AT-ATs were not the primary source of the snowtroopers that invaded the base and were more the blitzkrieg force, followed up by troop transports/etc, or they could offload their troops somehow. It's not clear from ESB how the timing of Vader and other soldiers being on ground works so it's safe to say this is inconclusive.
#Endor
An AT-AT briefly appears on Endor, too, right in front of some weird ramplike things at a coincidental height:
A more compelling image is what I believe is a fan-made image from Endor, showing an AT-AT "docked" to the shield installation.
It's not conclusive what purpose those ramps serve in the movie. It's possible they are platforms to shoot from, though these would be poor given the lack of cover. It is far more likely to be a rendering of a gangway modeled after something like:
This seems to be a pretty strong implication that a common loading technique for an AT-AT is through a gangway (and not crouching). But it also does not conclusively show that the walker must load through a gangway.
#Jakku
I cannot find a screenshot from TFA, but there is a crashed AT-AT next to the Star Destroyer. This is inconsequential to your question but included here for completeness.
#Conceptually
Given that the movie evidence is inconclusive..
AT-AT stands for All-Terrain Armored-Transport. Conceptually, it makes nearly no sense for an AT-AT to not be able to deploy its troops somehow (especially given the ridiculous height requirements for either a ladder or other method of deploying the troops).
Now... you would think that if you design a transport for assault such as an AT-AT that it wouldn't require specialized docking stations or other equipment specifically to offload them (such as the Endor station).
For that reason it seems plausible that the design of the AT-AT would include either:
- Easy way to offload troops (ropes, ladders, whatever)
- Ability to kneel
Offloading troops through a ladder down many meters seems less than a safe way and if I was designing a vehicle like an AT-AT to be so heavily armored, it seems the best way to make it safer is to lower the AT-AT itself as it will provide considerable cover at lower heights too.
#Legends Canon
In Legends, it is 100% possible to kneel an AT-AT. There is a short story in Tales from the Mos Eisley Cantina called When The Desert Wind Turns which covers a stormtrooper, Davin Felth, who performed this technique when unexpectedly under attack in a training environment. See ATAT scenario there for the full story, but basically he knelt the walker to deal with potential for being tripped.