Document the system database

[ericharmeling]

Eric Harmeling (ericharmeling) commented:

We currently don't document the system database very well.

AFAICT, the only place we explicitly define the system database in our docs is on the SHOW DATABASES page. This is insufficient, especially if a simple SHOW statement surfaces the table.

The naming conventions for our preloaded schemas/databases (especially for the system database and the crdb_internal virtual schema) are misleading, which could add to the confusion around the purpose and contents of the system database.

We should do the following:

• Define the system database in a dedicated section on preloaded databases. This would fit nicely under a new "Preloaded databases" section on the Name Resolution page.
• Make a note on the System Catalogs page that the best way to query system metadata is through our system catalog tables and views, and not through the system database.

We should not do the following:

• Define each table in the system database.
• Create any new examples that query the system database.

Jira Issue: DOC-1102

[ericharmeling]

@sheaffej