code insights: make internal/db/schemadoc generate separate files per DB

[emidoots]

(See #17218 for general plan / explanation of the work being done on code insights.)

For implementing RFC 273: Code Insights backend, we will be adding a separate TimescaleDB database deployment.

Currently schemadoc assumes all databases should be added to the same schema doc. This works for codeintel because it specifically aims to be compatible with the frontend DB (putting codeintel and frontend in the same DB is possible, though not advised.)

In contrast, TimescaleDB will need to be deployed separately. Although it is just Postgres, it often lags a bit behind in version (current 12 vs Postgres 13) so we will be deploying it separately instead. Thus, we should update schemadoc to generate a separate documentation file pr DB.

[github-actions]

Heads up @Joelkw @felixfbecker - the "team/code-insights" label was applied to this issue.