[Doc][FSDP] Currently FSDP doc is missing in the repo

[PopSoda2002]

Context: CN, EN

Should add quick start, parameter table etc. in the repo's doc, maybe place like: https://github.com/THUDM/slime/blob/main/docs/en/get_started/usage.md

[lin0303-siyuan]

I will take this.

[PopSoda2002]

Solved by #1092!