[docs] Update new builder instructions to default to building each change

We generally prefer build bots which build each change as they come in as long as they can keep up. This generates much higher quality feedback to a developer. However, the code structure is such that we currently default new builders to batching.

This change updates the getting started instruction for a builder to explicitly advocate for not collapsing requests.

Differential Revision: https://reviews.llvm.org/D112776