docs: add link to Discussions (#2405)

* Add link to Discussions in issue template chooser

* Add link to discussion to website footer

* Add Discussions badge to README

* Add link to discussion to CONTRIBUTING.md

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,4 @@
+contact_links:
+  - name: Discussions
+    url: https://github.com/KaTeX/KaTeX/discussions
+    about: Please ask and answer questions here.

CONTRIBUTING.md

@@ -2,7 +2,7 @@
 
 We welcome pull requests to KaTeX. If you'd like to add a new symbol, or try to
 tackle adding a larger feature, keep reading. If you have any questions, or want
-help solving a problem, feel free to stop by our [gitter channel](https://gitter.im/KaTeX/KaTeX).
+help solving a problem, feel free to stop by our [GitHub Discussions](https://github.com/KaTeX/KaTeX/discussions).
 
 ## Helpful contributions
 

README.md

@@ -2,7 +2,7 @@
 [![npm](https://img.shields.io/npm/v/katex.svg)](https://www.npmjs.com/package/katex)
 [![CircleCI](https://circleci.com/gh/KaTeX/KaTeX.svg?style=shield)](https://circleci.com/gh/KaTeX/KaTeX)
 [![codecov](https://codecov.io/gh/KaTeX/KaTeX/branch/master/graph/badge.svg)](https://codecov.io/gh/KaTeX/KaTeX)
-[![Join the chat at https://gitter.im/KaTeX/KaTeX](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/KaTeX/KaTeX?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![Discussions](https://img.shields.io/badge/Discussions-join-brightgreen)](https://github.com/KaTeX/KaTeX/discussions)
 [![Dependabot Status](https://api.dependabot.com/badges/status?host=github&repo=KaTeX/KaTeX)](https://dependabot.com)
 [![jsDelivr](https://data.jsdelivr.com/v1/package/npm/katex/badge?style=rounded)](https://www.jsdelivr.com/package/npm/katex)
 ![katex.min.js size](https://img.badgesize.io/https://unpkg.com/katex/dist/katex.min.js?compression=gzip)

website/core/Footer.js

@@ -54,7 +54,7 @@ class Footer extends React.Component {
                 <a href={this.pageUrl('users.html', this.props.language)}>
                   Who is using KaTeX?
                 </a>
-                <a href="https://gitter.im/KaTeX/KaTeX">Gitter Chat</a>
+                <a href="https://github.com/KaTeX/KaTeX/discussions">GitHub Discussions</a>
                 <a
                   href="https://stackoverflow.com/questions/tagged/katex"
                   target="_blank"