Fix various broken links and out-of-date text

[WardBrian]

Submission Checklist

• [x] Builds locally
• [x] New functions marked with <<{ since VERSION }>>
• [x] Declare copyright holder and open-source license: see below

Summary

While reviewing the site in dark mode to make sure it all looked correct, I found a slew of either broken links or statements which were missed in various updates (mostly related to tuples)

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):

Simons Foundation

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC BY-ND 4.0 (https://creativecommons.org/licenses/by-nd/4.0/)