Document notes partitioning design discussions
What does this MR do and why?
notes table is one of the largest tables in GitLab.com's production database and its size must be reduced.
This MR adds a document that summarizes the relevant discussions, the decisions and the information gathered from &10886 and #416127 (closed) inspired by https://docs.gitlab.com/ee/architecture/blueprints/ci_data_decay/pipeline_partitioning.html.
Edited by euko