Improve user experience with explanatory text and page structure

[matt-dray]

It would be helpful to provide explanatory text to explain the purpose of each page and how to use it. For example, it's not clear that you need to click a cell in the table to produce a waffle chart.

Adding subheaders in each tab may also be useful. For example, in the 'Summary Table' tab, you could have three sections: 'Select inputs', 'Summary table' and 'Publication listing', or something.

It may also help to move the 'additional variables' dropdown to a position just above the publications table, so it's purpose is clearer.

[matt-dray]

• Consider bs4Dash::box() to house each section (dropdowns in one box, output table in another, etc)
• Remove 'debug info'
• Maybe add page titles and section headers
• Adjust layout for ease of use/clarity
• fluidRow() for the dropdowns