Add previous/next buttons on bottom of section pages

[maxpinheiro]

now that we've split the computer vision module into separate sections on different pages, we should be able to navigate between the sections from each page

• first: add two new fields to the module JSON objects called "prevUrl" and "nextUrl" that contain the urls of the previous and next sections. the fields should be empty strings if there is no
• then: update the Module type in ModulePage.tsx and have the component render links/buttons at the bottom of the page