[Parent] Improve documentation section that covers "Extend Kedro"

[stichbury]

Child of #2799 This is a parent itself and will cover all the changes necessary to popular pages in the "Extend Kedro" section (namely plugins). Won't be addressed until 0.19.0 and is relatively low priority compared to #3200 but higher than #3201.

Parent ticket because there are already a bunch existing for plugins docs improvements, so this is the umbrella to make sure they're all addressed as part of the improvement.