Add links to index on function definitions

[WardBrian]

Submission Checklist

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

Summary

Tries to help with #802 by adding a link to all functions to their section in the alphabetical index:

This also improves the content of the index to better indicate where you will be going, much like a real book index lists the page numbers:

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]

looks great and builds on my machine - many thanks!

[avehtari]

Awesome!