Introduce spread (...) more deliberately in various Bicep docs pages

[anthony-c-martin]

As per feedback under #15451 (comment):

"The spread operator isn't well known or difficult to discover"

IMO, yes. I didn't know spread exists until just now; I had no trigger to think it might be used to solve this problem. When searching the internet, I did land on the "set or skip" github issue. I did not land on this particular example in the bicep spread documentation page.

And:

Maybe the bicep spread docs can be modified to more explicitly showcase the "conditionally set property on object" / "conditionally add element to array" usage pattern? I.e. make it more discoverable via internet search (SEO).
https://learn.microsoft.com/en-us/azure/azure-resource-manager/bicep/operator-spread

Maybe add a note to the docs pages for if and ternary ? :, mentioning that you cannot use them for these use cases, that you need to use ... spread instead?

It sounds like we have some work to do on making this more discoverable and searchable in the Bicep docs. Creating this issue to track that.