Docs model ladder

[IanMagnusson]

Trying to add some documentation for the model ladder (per #683) as I figure out how to use it.

[IanMagnusson]

Ideally we don't have to use --s3 for the usage example, but weka seems to be broken for a number of reasons right now.

1. the current dolma data mix with the flan with rules based sep is not in weka
2. If I revert to an earlier version of the ladder that didn't have this version of flan it still crashes without finding training data in weka although that data is there when I check manually.

[IanMagnusson]

Would be nice to also have the -h response for the other commands in the script (is there an easier way to get these than to do a gantry command to run this with the env all set up. When I try to run it locally I get env issues even tho I have OLMo pip instal -e

Likewise it would maybe be nice to list currently available sizes, lengths, and data mixes.