Add the [params] concept to front matter

[bep]

This is deliberately very simple, but should not break anything. We need to introduce this in baby steps, but this should allow us to introduce this in the documentation.

Note that the params section's key/values will be added to .Params last. This means that you can have different values for "Hugo's summary" and the custom ".Params.summary" if you want to.

Updates #11055

[bep]

@jmooring what do you think? Note that I have deliberately not tried to solve all of the related issues (e.g. taxonomies).

[jmooring]

Findings...

• 👍 Taxonomies works both in root and under params.
• 👍 When key is duplicated in both root and params, params wins

Should we throw an error or warning if key is defined in both root and under params? Conceptually it's a duplicate. I believe we normally rely on the unmarshaler to handle that for us, so I guess this would be some extra work.

[bep]

Should we throw an error or warning if key is defined in both root and under params?

For the short term, I think we say that that's a feature and not an error situation (as my summary example kind of demonstrates). I have left the original issue open, though.