examples.html: update gists

* Add "$schema" everywhere.
* Add "description" as well.
examples.html: more descriptive introduction to the first schema
Minor changes

* Add a supposedly missing <em></em>.
* Fix a little point of grammar.
* Fix an extraneous space.

Minor, I tell you.
examples.html: add two constraints to tags

Require that there be at least one tag, and that all tags must be unique.
Merge pull request #55 from fge/gh-pages

Some minor updates to the example page