Fix "Add your org" link, hide "Add your testimonial" one

[huonw]

This fixes two links that were previously explicitly pointing at the 404 page, after the migration from the old site:

• the "Add your organization or company" link on the user spotlight page, which used to point to https://pantsbuild.org/register : this still works (appears to be 301 redirect configured in CloudFlare?), and redirects to a GitHub issue template.
• the "Add your testimionial" link on the testimonials page, which didn't previously exist (https://web.archive.org/web/20230604011655/https://www.pantsbuild.org/docs/testimonials). I've hidden this one to at least not have a wildly broken link in people's face, but we can restore it once we have an appropriate process. (Although I imagine most people looking at the testimonials page will be new users, i.e. not in a position to write a testimonial yet.)

This was noticed by a user on slack: https://pantsbuild.slack.com/archives/C046T6T9U/p1735280301069719. Thanks!