feat(docs): Improved contributing guide

[gabrielmfern]

This new contributing guide includes a lot of changes and a lot of additions. First it changes
the CONTRIBUTING.md to be a pointer into the contributing guide in the docs because of its new size
and level of detail.

Secondly, instead of having one page for the contributing guide, there's a new section for it
with its pieces inside.

The new contributing guide includes two extra things that I've found are helpful, that other
projects include it themselves:

1. A codebase overview
   • Mentioning how we do things, what tools we use and how our files are structured
2. A step-by-step of the development workflow,

As for the two sections that we had before for Opening issues and Opening pull requests,
they are now pages in the docs. To them, I've also added things that I think are really
important, that might guide people coming across it to do better work.

Opening issues section

Codebase overview section

Opening pull requests section

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
react-email	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 15, 2024 2:31pm
react-email-demo	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 15, 2024 2:31pm

[gabrielmfern]

The staging deploy also worked out, if anyone wants to check it out https://react-email-feat-improved-contributing-guide.mintlify.app/contributing/opening-issues.