### What changes were proposed in this pull request?
Move Rest API out from monitoring.md to webapi.md
### Why are the changes needed?
To close CELEBORN-1436
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
Review https://github.com/turboFei/incubator-celeborn/blob/webapi_md/docs/webapi.mdCloses#2624 from turboFei/webapi_md.
Lead-authored-by: Wang, Fei <fwang12@ebay.com>
Co-authored-by: Cheng Pan <pan3793@gmail.com>
Signed-off-by: Cheng Pan <chengpan@apache.org>
