docs: showcase contribution button points to the Docusaurus repository

What happened

The website's “Are you using this? Add your company” button in website/pages/en/users.js links to:

https://github.com/facebook/docusaurus/edit/master/website/siteConfig.js

That is the Docusaurus repository, not twitchtv/twirp. A user following the button would edit or propose a change against the wrong project.

Suggested fix

Point the button to the Twirp repository's actual showcase/configuration source, or replace it with the project's documented contribution path. Please verify the link from the deployed “Who's Using This?” page.