Give some basic examples to illustrate content processing flow in devdocs

[raysonkoh]

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The content processing flow was updated in the huge V3 release (#1403). While the devdoc does provide a high-level idea of the content processing flow, I think it would be good to have some basic examples to illustrate the flow more concretely.

Describe the solution you'd like

Maybe one example could be a simple markbind project such as the template, and another could be more advanced that covers some edge cases.

It doesn't have to be super low-level, but it should at least cover, given a source file (index.md), what are the key transformations at each step of the content processing flow (nunjucks -> markdown -> html).

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