DOC: Add project details page with TOC that includes background + citations

[NickleDave]

• change main index to have grid of boxes like VocalPy + AstroPy, et al.
• add a box to the grid for "Project Details" like AstroPy that links to a project details index page
• The Project Details index should have a table of contents with two pages: background / about "vak" (current "about vak page) and "papers that use vak" (or some title like that)
• the latter should copy refs from the current "about vak" page--there's a section there on "papers that use vak"--but I think we should if possible have the whole page be a list of citations, not the weird thing I do now with one sentence that has a bunch of citations, that are links to footnote-like refs in a "references" section