Dashboard: add text

[laurasootes]

Add some more text to make the use of the dashboard and the examples a bit clearer. Specifically:

• [ ] Landing page: rename “pages”, and clarify what images/text/timeseries means in this context
• [ ] Landing page: what is the dashboard
• [ ] Examples: explain what the example is: what is the data, what is the task, what do the colours mean
• [ ] Page titles: make page titles more informative than simple “Image explanation"

Suggestion from Sarah to remove the pages section fully