Add & clarify instructions for appendices, figure credits, supplemental material

[codydunne]

In part re: the comments on the original Google Drive file.

See the attached tvcg-journal-latex-template-2023-01-04.pdf and the updated tvcg-journal-latex-template-2023-01-04.pdf on Google Drive.

See the attached tvcg-journal-latex-template-2023-01-04b.pdf and the updated tvcg-journal-latex-template-2023-01-04b.pdf on Google Drive.~~

[mattbrehmer]

tested Jan 10 in Overleaf