docs: add guide "using `etl` utils" (with examples, code snippets, etc)

[lucasrodes]

One-liner

We have multiple tools in etl and owid.catalog ready to be used, but they are not visible to users.

Context & details

As we develop and add more functionalities to our libraries, it is hard to track all of these. We should have a space in our docs to list the most commonly used code snippets.

I propose starting with a guide named "using etl utils" (or something similar). In this section, we should have various subsections with these common usages. Some candidate subsections could be:

• Correctly aggregate data (continents, World, world bank income, etc.)
• Copying metadata between indicators, tables, etc.
• Log your script (paths.log)

Feel free to add other actions that you think should have a place in this guide.

Deadline

I will work on a first draft at some point during 2024Q3 (maybe cooldown).

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