📜 sections: Table types, ETL tips

[lucasrodes]

This PR consolidates changes from:

• https://github.com/owid/etl/pull/3572: Add documentation on typing in ETL.
• https://github.com/owid/etl/pull/3573: Add a section to our docs for general ETL tips.

The reason for this was unexpected errors in CI/CD that were preventing these PRs to pass all tests.

[owidbot]

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```diff Legend: +New ~Modified -Removed =Identical Details Hint: Run this locally with etl diff REMOTE data/ --include yourdataset --verbose --snippet ``` Automatically updated datasets matching _weekly_wildfires|excess_mortality|covid|fluid|flunet|country_profile|garden/ihme_gbd/2019/gbd_risk_ are not included

Edited: 2024-11-20 10:55:08 UTC Execution time: 3.86 seconds