doc/dev/ci: add mention of build failure logs

Created by: bobheadxi

We have this somewhat spread across here and handbook - I think it kind of makes sense in the handbook given it's Sourcegraph-teammate-specific, but for now it's not a lot of content to reference in both places so I'm just going to have it in both. Part of https://github.com/sourcegraph/sourcegraph/issues/37617

Handbook counterpart: https://github.com/sourcegraph/handbook/pull/4016

Test plan

n/a