nit: Remove busy waiting on scheduler

[rkooo567]

Motivation

This PR includes 3 changes.

• contributor guide has no instruction regarding build. I found it is in install page, and I added a link there.
• Use setproctitle to change the process title for sglang. (it is contraversial, and we need to introduce a new dependency).
• Scheduler has busy waiting on recv_pyobject even when there are no requests which makes scheduler have 100% cpu usage even when the engine is idle. I make it use poll API when there's no batch to run (we need busy waiting when there are batches because we need to keep running batches).

Modifications

Checklist

• Format your code according to the Contributor Guide.
• Add unit tests as outlined in the Contributor Guide.
• Update documentation as needed, including docstrings or example tutorials.

[rkooo567]

I just made a PR to start contribution. Not sure if change makes sense to sglang perspective...

[ByronHsu]

setproctitle looks pretty useful!