Amphtml: Cleanup amp-gist experiment

[aghassemi]

• [ ] Remove experiment entry and code guard
• [ ] Update documentation
• [ ] Add AmpByExample.com sample ( or update it to reflect that it is not experimental anymore if sample was already created )

[aghassemi]

/cc @levidurfee Mid April sounds reasonable for taking amp-gist out of experimental and launching it.

[levidurfee]

/cc @aghassemi I'll take this out of experimental later today.

[aghassemi]

@levidurfee Thanks. No rush though, validator rules have not made it to production yet (will be in probably next week). So until amp-gist shows up as valid on https://validator.ampproject.org/ it needs to stay experimental. (You can submit the PR though, we just won't merge it until next version of validator is pushed to production)