docs: external postgres

[oleobal]

Add a how-to page on using an external database.

The new page is called external-database but maybe it should just be external-data since we might put the external S3 instructions on there as well (once that's done).

Companion PRs:

• https://github.com/Substra/orchestrator/pull/210
• https://github.com/Substra/substra-backend/pull/658

Fixes FL-649

[linear[bot]]

FL-649 Allow using external PSQL on backend and orc

Context and user need: Cornell request: they want to use their own PSQL database instead of the ones packaged with Substra components Functional spec: Today PSQL address and credentials are hard coded in the charts template. Technical spec: Parametrise the connexion address and credentials to allow other db. Interesting lin: [https://www.postgresql.org/docs/7.3/pygresql-pg-pgobject.html](https://www.postgresql.org/docs/7.3/pygresql-pg-pgobject.html) Acceptance criteria: Can use an external postgres