docs: new section for contributing (#11812)

## Description

As per [rfc/29](gatsbyjs/rfcs#29), this PR introduces a major change to the Gatsby docs: a new `/contributing` section. I did a big find and replace to update all links, and there are also redirects for pages that moved.

This is a pretty large PR, and I welcome any and all feedback on the new structure and content. My only request is that we try to resolve any feedback quickly so that community PRs aren't working on outdated stuff.

Here's a screenshot of the new Contributing landing page:

![contributing landing page](https://user-images.githubusercontent.com/1045233/52890806-938f8a00-313b-11e9-8645-c4991fc95fa3.png)

## Related Issues

Fixes #11803, Fixes #8847

## Outstanding questions
- Do we need to make any additional changes to CODEOWNERS?
- I included a link to `/contributing` in the docs sidebar to ease the transition. Should the link use some kind of icon to distinguish that it takes the user away from the docs?
- Should we add a new icon to the mobile nav (I didn't make any mobile changes in this PR)?
- Any changes to the sidebar, specifically around accordions?

## Subsequent related issues/stubs

- Gatsby Governance Model: gatsbyjs/gatsby#11805
- Triaging GitHub Issues: gatsbyjs/gatsby#11810