Update html guidelines (#1184)
Update html guidelines (#1184)

This PR updates the contribution docs for HTML examples, `CONTRIBUTING-HTML.md` to add more detailed and (hopefully) useful guidelines.

It restructures that file into 3 main sections:

1. a walkthrough of how to add an example
2. a set of guidelines for writing good examples
3. a bunch of extra bits and pieces (changing editor height, encoding entities, fonts)

The set of guidelines is taken from the gist at https://gist.github.com/wbamberg/355a58406c01dc9ec3629fe0c69ff580, with some small updates based on review comments.



mdn/interactive-examples,mdn/interactive-examples,mdn/interactive-examples