Admin console - NXQL query es reindexing explainer

Explainer and reference point for ES reindexing
[EXPLAINER-ES-REINDEXING-NXQL]

AC
Explanation of impact:

• The query could impact performance if it involves high volumes

Reference point: 2000

The number 2000 being the lowest point reached in our continuous benchmarks during 2022/2023 (2210, see https://benchmarks.nuxeo.com/continuous/platform-137/index.html#reindex) rounded down to the closest thousand when using MongoDB. It is also close to performance attained when using a relational database in previous benchmarks, which makes it a safe reference point.