cweb #10129908 ui: side bar to explore HTML pages [open]

Create a sidebar provided by the extension as detailed in https://developer.mozilla.org/fr/docs/Mozilla/Add-ons/WebExtensions/user_interface/barres_laterales

Move to this sidebar the parts of the UI related to the data detected in the HTML page.

Add links to documentation hosted on cubicweb.org that explains how to make the server behave better (fix bad content type, fix no content negociation, fix link rel= in html header, etc)