Update Jinja2 Docset

I apologize for pushing an update this early and not having double checked the docset before. Here a list of what the update is all about:
* `info.plist` has been updated in order to get table of content support.
* All titles of the documentation pages have been stripped down to the essential (no reference to docset name or version, as suggested on the [Docset Contribution Checklist](https://github.com/Kapeli/Dash-User-Contributions/wiki/Docset-Contribution-Checklist)).
* The documentation layout has been improved (no more horizontal scrolling, as suggested on the [Docset Contribution Checklist](https://github.com/Kapeli/Dash-User-Contributions/wiki/Docset-Contribution-Checklist)).
* The readme has been slightly updated.