docs: add PostgreSQL coverage to installation page

[dimitri-yatsenko]

Summary

• Documents the datajoint[postgres] extra and the PostgreSQL Docker setup so the Installation how-to matches multi-backend support, not just MySQL.
• Reframes the page from "MySQL-compatible" to MySQL/PostgreSQL as peer backends, and expands the cloud-database list (RDS, Cloud SQL, Azure Database) to include PostgreSQL.
• Adds version notes: PostgreSQL backend introduced in 2.1, database.name setting introduced in 2.2.1. Adds a psycopg2 troubleshooting subsection and a pointer to configure-database.md#postgresql-backend for the full reference.

Test plan

• mkdocs serve and open /how-to/installation/ — confirm both Docker snippets render, the datajoint[postgres] extra is visible, cloud DB list shows PostgreSQL, and the psycopg2 troubleshooting subsection appears.
• Click the configure-database.md#postgresql-backend link and confirm it lands on the right anchor.
• Re-read for peer framing — PostgreSQL should never be described as an "alternative" backend.

[MilagrosMarin]

Thanks @dimitri-yatsenko! Verified the version claims and packaging end-to-end:

✅ datajoint[postgres] extra exists in datajoint-python/pyproject.toml: postgres = ["psycopg2-binary>=2.9.0"]
✅ PostgreSQL in 2.1 — confirmed via git tag --contains on the PG-compat commits in datajoint-python (first land in v2.1.0).
✅ database.name in 2.2.1 — confirmed: commit 58de0f32 feat: add dbname setting for PostgreSQL connections first appears in tag v2.2.1.
✅ configure-database.md#postgresql-backend anchor exists (line 129).
✅ PostgreSQL 15+ / MySQL 8.0.13+ are consistent across CONTRIBUTING.md, src/about/whats-new-21.md, src/how-to/testing.md, src/how-to/configure-database.md, and the new installation page.
✅ Cloud DB expansion accurate — RDS Aurora MySQL/PostgreSQL, Cloud SQL, Azure Database all support PostgreSQL.

Two issues worth addressing — both small but visible to readers:

1. The linked target still uses the "alternative" framing this PR explicitly rejects. src/how-to/configure-database.md:131-132:

!!! version-added "New in 2.1"
    PostgreSQL is now supported as an alternative database backend.

A reader follows the new peer-framed link from installation.md (line 57) → configure-database.md#postgresql-backend → and lands on "alternative database backend" immediately. Suggest reframing the admonition body in the same PR, e.g.:

!!! version-added "New in 2.1"
    PostgreSQL is supported as a peer database backend alongside MySQL.

2. The new GRANT pointer is dangling. Line 129 of installation.md:

PostgreSQL uses a different `GRANT ... TO username` syntax — see [Configure Database Connection](configure-database.md#postgresql-backend) for backend-specific guidance.

The #postgresql-backend section (lines 129–220 of configure-database.md) covers configuration, env vars, programmatic config, docker-compose, and backend compatibility — but has no GRANT / permission content. A reader following this link will not find what they were promised.

Two reasonable fixes:

• Drop the link and just inline a one-line PostgreSQL example, e.g. GRANT ALL PRIVILEGES ON DATABASE my_db TO username;
• Or add a "Database permissions" subsection to configure-database.md and point there.

Optional nit: Worth noting briefly that pre-2.2.1 PostgreSQL connections were limited to the default postgres database — that's the context the database.name setting unlocked. Not load-bearing, just a sentence.

Otherwise this is a clean, well-scoped expansion. Happy to approve once the two pointer issues are sorted.