Put examples in Glean Dictionary

[chutten]

In reviewing the pingcentre-sent data in bug 1820550 I found their data dictionary and events docs very helpful in no small part due to the constant use of examples.

The Glean Dictionary should totally steal that idea-- I mean, expand on that success.

I'm not sure how. Static examples could be difficult to keep up-to-date. But maybe we could feed in from the Data Platform some quick stats of "in the past six months, this field has only ever contained the value CLICK" or "most common values are 12, 14, and 256"? I dunno. I just like examples.