Add headers to the vision section

[epec254]

I added headers here since I think they increase the readability / scannability of the section for a casual reader looking "just to get the gist"

[tdcox]

Thanks. We try to avoid merely stylistic changes to long-running sections of the document, since this just makes it harder for people to see where there have been material updates. The focus is really on ensuring that we are keeping abreast of developing challenges in the space.