Improve documentation for menu rendering.

[LiaungYip]

Fixes #1393

[LiaungYip]

Hopefully the examples I added will help new users trying to build nested menu templates.

I tried to cover the things which confused me when I was learning it:

• Using the HasMenuCurrent() and IsMenuCurrent() functions to style the active menu items;
• Using recursive partial templates to render nested menus;
• Only rendering the sub-menus relevant to the current page.

Note: I tried to sign the project CLA, but https://cla-assistant.io/spf13/hugo currently gives an error.

[LiaungYip]

menutest.zip

A small Hugo site illustrating use of the menu functions, on nested menus.

[bep]

This is very, very good.

A little note about the recursive part. I just fixed a recursive bug reported by a user that used inline template definitions to recursive render a menu, see the link from the issue linked in here:

bdc02ed

[LiaungYip]

Hi @bep , is this good to merge or did you want further edits?

[bep]

No, it looks very good to me -- but I try to delegate doc merges to @digitalcraftsman

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