docs(cdk-experimental/menu): add examples to menu documentation

[andy9775]

I'm not 100% sure if I connected the examples with menu.md correctly. PTAL.

[jelbourn]

For the examples, we try to prefix all of the styles with .example- so it's clear where they're coming from

[andy9775]

Those are the classes that the cdk directives add.

[jelbourn]

Yeah, we can either add additional class in the example, or put all of these as child selectors, e.g.

.example-menu { ... }
.example-menuitem { ...}

/** OR **/
.example-menu .cdk-menu {
  ...
}

.example-menu .cdk-menuitem {
  ...
}

[andy9775]

Ah, ok. I see what you were saying now.

[andy9775]

@jelbourn Done

[jelbourn]

I don't think the alpha notation here works in older browsers

[andy9775]

@jelbourn feedback should be addressed.

[jelbourn]

LGTM

(I was on vacation for a bit)

[andy9775]

@jelbourn no problem.

Hope your vacation was good.

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