Docs: Configuration file pages as explanation and reference (Diátaxis)

[benjaoming]

This PR proposes to:

• Shorten Explanation top-level: In order to not make the Explanation top-level yet another item longer, we add the new subsection "Deep-dive into Read the Docs" (this which was already in the cards)
• Change the general config-file/index to an explanation page (we could refactor the location, but in the special case of config-file contents, we currently have it all in this folder)
• We drop the intermediate step config-file/index that's currently making users go through an unnecessary intermediate step

Might wanna add a note about the upcoming requirements to have a configuration file w/ a link to the blog post? Or do that later?

Fixes: #8648

Refs: #9747

This PR supersedes #9921

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📚 Documentation previews 📚

• User's documentation (docs): https://docs--10416.org.readthedocs.build/en/10416/

• Developer's documentation (dev): https://dev--10416.org.readthedocs.build/en/10416/

[ericholscher]

Did some content updates here, I think we're close..

[benjaoming]

@ericholscher what do you think about adding a small section about configuration files and implicit defaults, i.e. the topic of using latest for build images? I think this could be a good location to elaborate why it's not a supported feature. I'm thinking that having a reusable reference is valuable + consolidating knowledge from detailed discussions.

[ericholscher]

@ericholscher what do you think about adding a small section about configuration files and implicit defaults, i.e. the topic of using latest for build images? I think this could be a good location to elaborate why it's not a supported feature. I'm thinking that having a reusable reference is valuable + consolidating knowledge from detailed discussions.

I think we can tackle that later. I don't want to expand the scope of this PR, so we cna can get it merged.