In Revenge of the Sith, according to the title crawl, Anakin and Obi-Wan "lead a desperate mission to rescue the captive Chancellor." They do this by arriving in two one-man starfighters. Grievous' flagship appears to only hold droid fighters, so they couldn't have stolen other ships once there, and they probably didn't originally intend to crash-land the entire flagship. Do any sources explain how they originally planned to get Palpatine off of the ship?
The novelisation covers this. They were hoping to secure a shuttle in the Hangar bay.
Anakin slewed his starfighter into a landing that scattered droids with the particle blast from his sublight thrusters and for one second he was nine years old again, behind the controls of a starfighter in the Theed royal hangar, his first touch of a real ship’s real cannons blasting battle droids - He’d have done the same right here, except that Palpatine was somewhere on this ship. They just might need one of the light shuttles in this hangar to get the Chancellor safely to the surface; a few dozen cannon blasts bouncing around in here could wreck them all.
This he’d have to do by hand.
And assuming that plan failed (which it did), they had a backup;
“This is not the best plan we’ve ever had!” he shouted. “This was a plan?” Palpatine sounded appalled.
“We’ll make our way forward!” Obi-Wan shouted. “There are only droids back here! Once we hit live-crew areas, there will be escape pods!”