Add Section Headers to Lecture: What Are Some Common Relational Databases, and How Do You Install and Use Postgres?

Part of #69286

## Summary

The lecture challenge **"What Are Some Common Relational Databases, and How Do You Install and Use Postgres?"** is ~937 words with no section headers, making it hard to scan.

- **File:** `curriculum/challenges/english/blocks/lecture-working-with-relational-databases/687ea86615674e19296a78ce.md`
- **Word count:** ~937

## Acceptance Criteria

- Add `##` section headers to break the lecture body (the `# --description--` section) into logical sections
- Headers should reflect the natural structure of the content; no rewriting of existing prose
- See `curriculum/challenges/english/blocks/lecture-working-with-higher-order-functions-and-callbacks/673362d7f94d551edb532d24.md` ("How Does the Sort Method Work?") for the existing convention: short, Title Case `##` headers placed before each major sub-topic
- Do not modify the `# --questions--` section, code blocks, or front matter
- Ensure the lecture still reads clearly end-to-end after headers are added

## Want to Contribute?

This issue is part of the **Contributor Sprint** initiative! If you're interested in helping out:

1. Join our Discord: https://chat.freecodecamp.org
2. Claim the **Naomi's Sprints** role
3. Ping Naomi in `#naomis-sprints` to be assigned this ticket

Open curriculum/challenges/english/blocks/lecture-working-with-relational-databases/687ea86615674e19296a78ce.md and review the # --description-- section. Compare its structure with curriculum/challenges/english/blocks/lecture-working-with-higher-order-functions-and-callbacks/673362d7f94d551edb532d24.md, then add short Title Case ## headers before major sub-topics without changing prose, code, front matter, or questions. Done means the lecture is easier to scan and still reads clearly end-to-end.

Written by the indexing model from the issue text.