docs: explicit when key is dynamic in services

[Sayrus]

It isn't clear to all users that xxx - a map of the form key: value means that the user can specify their own key. This PR adds additional examples to help discern whether the key is statically defined by notifications-engine or defined by the end user.

This follows questions asked on #argo-cd-notifications

I also reflected this in argoproj/argo-cd: argoproj/argo-cd#10589

[codecov]

Codecov Report

Base: 49.21% // Head: 49.21% // No change to project coverage 👍

Coverage data is based on head (9fc466f) compared to base (4d8552b).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #104   +/-   ##
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  Coverage   49.21%   49.21%           
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  Files          31       31           
  Lines        1855     1855           
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  Hits          913      913           
  Misses        743      743           
  Partials      199      199           

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[kostis-codefresh]

Hello

Instead of changing the existing example, may I suggest you add a separate example that has multiple /spaces/channels/teams etc?

This way instead of having a single example that readers are expected to take and then comment/uncomment stuff, they simply can take the example that matches their case directly.