assets/webapp: Add UI guided tour to help users discover features

Add a Web UI guided tour implemented using the intro.js library.

The tour showcases the main browsing features offered by Software
Heritage web application by highlighting the relevant DOM elements
and displaying help texts in tooltips.

The tour can span multiple web pages and steps are configured in
a YAML file for help texts writing commodity.

Currently, the homepage, the main origin browsing view and content
browsing view are covered by the tour.

Related to T3202