Brief context / helptext on the landing page

[surchs]

On the landing page there is a short welcome text that covers the following points:

• This is a tool to annotate tabular data, e.g. from a BIDS dataset
• If you have a BIDS data dictionary, you can add it to make your life easier, but optional
• First we'll ask you to tell us what "columns" are about
• next we'll ask about values
• then you can download it all
• also: we're from Neurobagel, it's cool, check out the website