docs: add tutorial for building custom store engine

one interesting idea could be to use an excel sheet reader/writer library or sth similar to store the current values and the desired set as well. reconciliation would rewrite the excel sheet.

this way the tutorial could show that now we gained the ability to also reconcile different types of items into different excel sheets.

then we could demonstrate nested reconciler sets by adding another store engine that models a folder. so each item with lifecycle event could be sth that is stored in a separte excel sheet.

we could demonstrate that if we realize that instead of an API, we need to store the data in an SQL db. we can just swap the API implementation (proably we want to renmae the API to sth more generic, like remote-resource or sth)

The issue names no files or tests; start by locating the existing tutorial, store-engine implementation, and API abstraction. Done means a tutorial covering an Excel-backed store, nested folder-based reconciliation, and swapping the API implementation for SQL storage, with the intended abstraction name settled.

Written by the indexing model from the issue text.