Deduplicate index,overview.qmd files

[WardBrian]

Submission Checklist

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

Summary

This uses a bit of extra conditional logic to not need two separate main files for the pdfs and website. This also means the website uses the more traditional index.html for each main page.

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):

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/)

[mitzimorris]

just realized - conditional logic would be a natural way to add link to downloadable, versioned pdfs from index page.