docs: Make the baseof.html reference in the docs more obvious

[bep]

See https://discuss.gohugo.io/t/blocks-and-define-override-in-hugo-theme/3543

[marcojakob]

Yes, the use of block templates needs to be more clear in the docs:

• What's the meaning of baseof?
• Can I create multiple baseof files like baseof-blog.html or baseof*?
• Do I need to place the baseof files in the _default folder or could it also go into a section or any other folder?
• Where can I use block templates? It's not obvious why you can't use block keywords in partial files.

[bep]

This duplicates https://github.com/spf13/hugo/issues/2363

(@marcojakob I see some questions in your list that is better suited for the forum (like "what is the meaning of baseof?")

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