doc: Misc updates to codeintel development docs

Created by: efritz

• Fixes https://github.com/sourcegraph/sourcegraph/issues/23778.
• Fixes https://github.com/sourcegraph/sourcegraph/issues/18442.
• Fixes https://github.com/sourcegraph/sourcegraph/issues/18446.
• Fixes https://github.com/sourcegraph/sourcegraph/issues/18444.