Update documentation on extends

[FreekPols]

Included the use of extends for

• export to pdf, including all options in an export.yml file
• create a toc.yml file for a very long toc

[changeset-bot]

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[choldgraf]

I think the docs look good to me - it's great to improve the docs and discoverability around these workflows! I noticed it seems like we're updating a dependency here, is that intentional? Could you roll that back into a separate PR so we aren't doing too many things in one go?

[choldgraf]

Suggested change

	In some cases the ToC becomes very long, where it would be more convenient to create a separate ToC file. This can be done using [extends](#composing-myst-yml).
	If your Table of Contents is very long, it can make your `myst.yml` difficult to read and maintain. You can use [`extends:` functionality](#composing-myst-yml) to store the ToC structure in a separate file, allowing you to re-use it in a modular way.

[stefanv]

I was reviewing, when I realized that almost all the diffs I'm looking at are reformatting. But some do contain content diffs. It's probably best to separate these out in the future.

I'll continue reviewing later, skipping the formatting and focusing on the extends section.

[stefanv]

Unintended change?

[FreekPols]

This got in with a pull from angus somehow. Have discussed it with Chris already.

[FreekPols]

I was reviewing, when I realized that almost all the diffs I'm looking at are reformatting. But some do contain content diffs. It's probably best to separate these out in the future.

I'll continue reviewing later, skipping the formatting and focusing on the extends section.

Yeah, sorry, was reading the documents and thought it would be easy to format it all along. Not thinking about the review!