docs: show how to query a Job by id with a redis instance

[SAY-5]

Per samuelcolvin's note on #240: "yes it's very simple, you also need to pass a redis instance ... PR welcome to add this to the docs." Adds a short paragraph and an examples/job_by_id.py snippet under the existing "Job Results" section so users can look up a job from its id and see what happens when no such job exists (JobStatus.not_found).

Closes #240

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

@@            Coverage Diff             @@
##             main     #525      +/-   ##
==========================================
- Coverage   96.27%   95.85%   -0.43%     
==========================================
  Files          11       11              
  Lines        1074     1085      +11     
  Branches      209      144      -65     
==========================================
+ Hits         1034     1040       +6     
- Misses         19       24       +5     
  Partials       21       21              

see 1 file with indirect coverage changes

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5ee4b48...b85a993. Read the comment docs.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.