doc/dev: refresh CI dev docs with search notebooks

Created by: bobheadxi

Removes/refreshes outdated pipeline dev docs and refreshes it with snazzy new embedded notebooks! Most of this content are split out from the notebook I've been sharing around, which I've split into a multi-part notebook for ease of embedding and sharing. The goal is to provide a look at the internals of the generator for potential contributors.

Left step options as a "coming soon" - I will circle back to this.

Closes https://github.com/sourcegraph/sourcegraph/issues/30557

Test plan

sg run docsite

Then navigate to http://localhost:5080/dev/background-information/continuous_integration#pipeline-development