Add how-to guide for connecting to PostgreSQL via psql (#188)

## Summary
- Add new how-to guide (`connect-to-postgres.md`) with the `psql` command using `op read` for 1Password credentials
- Add "Database" section to the how-to index linking to the new guide
- Link the new guide from the PostgreSQL reference card's Related section

## Test plan
- [x] Verified `psql` connection works from gilbert using the documented command
- [ ] Review doc formatting and content

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/188

docs/reference/services/postgresql.md

@@ -55,6 +55,7 @@ Backed up via [[borgmatic]] `postgresql_databases` hook. Streams `pg_dump` direc
 
 ## Related
 
+- [[connect-to-postgres]] - How to connect via psql
 - [[miniflux]] - Feed reader database
 - [[teslamate]] - Vehicle data database
 - [[borgmatic]] - Database backup