Docs/contributing index to mdx (#19057)

* Converted pages/contributing/index.js to mdx

* Removed rundant h1, it is created by the template from frontmatter title.

* Renamed to index.mdx

Author: mi-na-bot

docs/contributing/index.mdx

@@ -0,0 +1,18 @@
+---
+title: Contributing to Gatsby.js
+disableTableOfContents: true
+---
+
+import EmailCaptureForm from "../../www/src/components/email-capture-form"
+
+Thanks for being interested in contributing! We're so glad you want to help!
+
+Below you'll find guides on the Gatsby.js community, code of conduct, and how to get started contributing:
+
+- [Community](/contributing/community/): Learn why you should contribute to Gatsby.js, the most effective ways to do so, and all about the benefits (did we mention the free swag?)
+- [Code of Conduct](/contributing/code-of-conduct/): Read about what we expect from everyone participating in Gatsby.js to make it the most friendly and welcoming community
+- [Gatsby Style Guide ](/contributing/gatsby-style-guide/): The art of contributing to Gatsby, a.k.a. the detailed requirements that will make it more likely your contribution is accepted with minimal changes
+- [How to Contribute ](/contributing/how-to-contribute/): How to get the most out of your Gatsby.js contributing experience, including GitHub management tips, setup instructions for docs and code contributions, and more
+- [RFC process](/contributing/rfc-process): Learn how the Gatsby.js team manages bigger changes, by way of a “Request-for-comment” process on GitHub
+
+<EmailCaptureForm signupMessage="Want to keep up with the latest tips &amp; tricks? Subscribe to our newsletter!" />