Add link to third-party `JavaScript (JSX).tmLanguage` in docs.
Don't update the docs version by default

This was leading to a lot of unnecessary churn in the config file since
different YAML versions were serializing differently.
Fix use of "it's" in docs
Fix typo in doc
Docs: Give headers ids for easy linking

This gives markdown headers an id so that we can link directly to
sections of our docs. This is better than the alternative of adding them
all ourselves.
Community round-up #8