[Blog] Rewrite the presets post's compound learning section

[peterschmidt85]

Follow-up to #4180.

The section was a session-by-session log. It now carries two points, each with its evidence in a blockquote:

• Learning compounds when sessions are linked, because each session inherits the previous best, the findings and the patches.
• The toolkit automates the routine and leaves the steering to the engineer, through the prompt.