Improve postgresql section

[igaray]

The section in foundations pertaining to Postgres should be elaborated with:

• a guided exercise to startup postgres via docker, connect to it via psql, load a database provided with a sql file in the repo, and run some queries with
• resources on troubleshooting common RDBMS issues
• resources on using EXPLAIN and interpreting its output
• a guided exercise on troubleshooting a problematic query with EXPLAIN