[DOCS] DSB-1009: Add Learn page for GX <> Airflow data pipeline tutorial

[rachhouse]

This PR adds a concise page to the Learn docs that links users to the https://github.com/greatexpectationslabs/tutorial-gx-in-the-data-pipeline tutorial repo.

• Direct link to Netlify preview page: https://deploy-preview-10534.docs.greatexpectations.io/docs/reference/learn/integrations/airflow_pipeline_tutorial/

I've also done a little cleanup and tense standardization for the tutorial TOC and landing pages.

https://greatexpectations.atlassian.net/browse/DSB-1009

• [ ] Description of PR changes above includes a link to an existing GitHub issue
• [ ] PR title is prefixed with one of: [BUGFIX], [FEATURE], [DOCS], [MAINTENANCE], [CONTRIB]
• [ ] Code is linted - run invoke lint (uses ruff format + ruff check)
• [ ] Appropriate tests and docs have been updated

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