Improving the onboarding experience for Docs Open source

[simpsoka]

This PR adds a few updates to two files that are all about onboarding for folks who are new to contributing to docs.github.com:

1. Readme.md: https://github.com/github/docs/blob/9f62787550f024d56dbfde3c1146ba8f96115e09/README.md
2. Contributing.md: https://github.com/github/docs/blob/9f62787550f024d56dbfde3c1146ba8f96115e09/CONTRIBUTING.md

The readme updates introduce a very simple get started and contribute flow. It highlights that you can start just from clicking a button on docs.github.com, and that contributions can be made without writing a line of code (issue or discussion)

If you want to do something a little more complex, then you’re directed to contributing.md. That file now has a Getting Started guide at the top (before the TOC, so you’re not distracted by other parts of the article that you might not need yet). The guide is all about the main flow of checking out issues, opening a PR, getting it reviewed, and then merged. It references information that’s further down in the doc.

I also removed the parts about git troubleshooting, and simplified that by including “tips for working with our codebase,” which links to a learning lab module about managing merge conflicts. Eventually, we will want to update the Getting started with Git and GitHub docs and link to them instead of Learning Lab.

[zeke]

Thanks for jumping in! This is looking good. I left some suggestions.

[janiceilene]

@simpsoka This looks wonderful, thank you so much for making these updates so quickly ⚡

[zeke]

[martinwoodward]

This is looking MUCH better now.

One nit - I still think this section isn't particularly helpful and you'd be better pointing them to somewhere in the docs on how to clone locally from the command line. The in-product instructions you get in the clone button are the most helpful thing as you don't have all that YOUR-USERNAME thing and we have to miss a bunch of stuff here anyway if we were going to cover in that much detail like create a branch, commit the changes, pushing the code (having set up SSH keys or credential helpers etc).

[simpsoka]

Thanks, @martinwoodward! Just made the following update to simplify things regarding fork instructions (also added a line about why this is useful for developers as they're making changes).