Add a general debugging guide to the troubleshooting doc

[baha-bouali]

Adds a "How to debug your Airflow deployment" section to troubleshooting.rst,
structured around Agans' nine rules of debugging (as popularized by David A.
Wheeler's essay), with an Airflow 3–specific action under each rule.

closes: #43892

---

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)
  Assisted by Claude Sonnet 5, every line reviewed by the submitter.

following the guidelines.

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.

[baha-bouali]

Hey @amoghrajesh, just gentle ping to check this when you have a chance. Thanks!