Be explicit about Central not currently being intended as a dashboard, db not being an interface

https://forum.getodk.org/t/odk-central-clarifications-required/30070/6

Start by reading the linked forum discussion to understand the requested Central and database clarifications, then search the documentation for the relevant Central descriptions. Update the appropriate documentation so it explicitly states that Central is not currently intended as a dashboard and that the database is not an interface; confirm the wording is clear in the rendered Sphinx docs.

Written by the indexing model from the issue text.