Emit a warning that can be turned off when overwriting built-in .Params values

[bep]

I have slept on this.

With the addition of a params section in front matter, we get a much better separation of Hugo configuration/metadata and user metadata.

Me do merge in the Hugo metadata keys that's not in the user defined map. Having a merged view of this is very convenient in some situations:

• Access via .Params, e.g. .Params.summary, .Params.tags.
• Used as keywords in Related.

But we can and should warn (but allow that warning to be turned off) when people do this and similar:

---
title: "Foo"
params:
   title: "Bar2"
---

Maybe not the greatest example, but is at least simple.

[github-actions]

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