Add EventCallback content to Components topic+sample

[guardrex]

Fixes #11243

Internal Review Topic (EventCallback section)

• For now, I focus on basic implementation. Let's add more detail where you think it's required from here.
• Interesting bits are in the EventCallback section (a subsection of the Event handling section) and the accompanying sample updates for that content, which run lines 325-371 of aspnetcore/razor-components/components.md. The section is linked via the Internal Review Topic link. The rest of the updates here are general updates, and we can review those bits on the docs side.
• Layouts! ... is there any way to use EventCallback/EventCallback<T> across a the component-layout barrier (i.e., affect the layout's objects from a component using the layout)?

[guardrex]

@rynowak Do u have time to look at the section on EventCallback?

If so, use the link ... it goes right to the important bits (the docs team can review the rest of this PR):

Internal Review Topic (EventCallback section)

This one should get u to the diff for line comments (lines 325-371):

https://github.com/aspnet/AspNetCore.Docs/pull/11924/files#diff-68aec1e5867e7fd4666f410fdf9b3a88

Nevermind, I'm good. 👍

[rynowak]

I read through this and everything here seems reasonable. if you want more stuff to mine, here's my original writeup on this topic dotnet/aspnetcore#6351

[guardrex]

@rynowak It is based on your remarks. I made it a little plainer on the latest commit.

@danroth27 It may require another touch (or two), but it seems to be heading in the right direction.

It's ready for another 👁️.

Also ... cleaned up that Parent component method name to just ShowMessage. That first name was a SillyRex™️ choice.