docs: Adds 'Using _index.md' page

There has been confusion regarding the use of _index.md pages following
the shift to 'Everything is a Page' in Hugo v0.18:

https://discuss.gohugo.io/t/hugo-0-18-ignores-index-md-in-the-section/4896

This has lead to attempts at clarification:

https://discuss.gohugo.io/t/-index-md-pages-in-section-taxonomies-etc-are-not-rendered-using-single-html-template/5466

There is also an open issue regarding improving the docs on this topic:

https://github.com/spf13/hugo/issues/2827

This commit adds a dedicated page explaining how _index.md files fit
into Hugo's template hierarchy, what they can contain and how to render
content from them onto the respective list/terms/home page.

What this commit doesn't do is address confusion over former behaviour
of the 'index.md' files (note the lack of _). Instead it focuses on
clarifying the new behaviour in the hope of rendering the old moot.

Fixes #2827

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