Contribution guide is lacking in repo-specific content

We do include some contribution information on the website here.

I don't think it's terribly helpful as is.

There's an issue capturing some complaints on the main repo, but if we have docs on the website we should probably track that portion here. Also that issue is too broad, and we should break down specific items.

I'm starting here by capturing feedback with checkboxes. These could be cards on a project board if we'd like to do it that way. Or individual issues. But for now... just need to capture the feedback.

When looking at contribution guides, here are some references:

• WinUI
  https://github.com/microsoft/microsoft-ui-xaml/blob/master/CONTRIBUTING.md
  https://github.com/microsoft/microsoft-ui-xaml/blob/master/docs/developer_guide.md

• RN
  https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md
  https://github.com/facebook/react-native/wiki/How-to-Contribute-Code

• Contibuting page is duplicated:

  • Wiki: https://github.com/microsoft/react-native-windows/wiki
  • Linked from main readme: https://github.com/microsoft/react-native-windows/blob/master/docs/contributing.md
  • This same content shows in website’s sidebar: https://microsoft.github.io/react-native-windows/docs/contributing

Of note, this doc isn’t great. It’s more “here’s how to use git”, not “here are the quirks to contributing to this repo”.

• Remove SourceTree recommendation (it doesn't seem to be used by most of our core maintainers)
• Replace the boiler plat "how to use git" content with links to existing resources/articles.
  • Feedback: "this was mostly useful because it mentioned ‘yarn change’ and linked to the branch setup page"

There’s also this page on the wiki that’s at least more specifically helpful: https://github.com/microsoft/react-native-windows/wiki/Setup

• Setup page isn't repo-specific
  Again it’s pretty standard git boiler plate of “how to work in a fork”, pretty much nothing specific to our repo. We should link to other guides.

  • Feedback: "this was valuable, hadn’t worked with forks before"

• Setup page is duplicated
  Note, duplicated here: https://github.com/microsoft/react-native-windows/blob/master/docs/branch-setup.md
  We should pick where these things live. It’s github convention to have a CONTRIBUTING.md, but as you’ll see on RN’s repo they farm the rest out to wiki.

• Building RNW article isn't in the sidebar
  This exists: https://microsoft.github.io/react-native-windows/docs/building-rnw
  Which is a spiritual successor to this: https://github.com/microsoft/react-native-windows/blob/0.60-stable/vnext/docs/BuildingRNW.md
  It is linked to from the "getting started" page, but not from the "contributing to" page, nor is it in the sidebar.

• Playground readme is out of date
  https://github.com/microsoft/react-native-windows/blob/master/packages/playground/README.md
  It has some broken links (pointing to vnext – can unbroken catch these?). And there’s no mention to how you need the sln param (although to be fair you’ll hit the error and know what to do).

• RNTester's relationship with RNW is complicated and undocumented
  There are proposed changes to how RNTester is managed here and here. In the meantime there's too much tribal knowledge.

Start by reviewing docs/contributing.md, the website contribution page and sidebar, and packages/playground/README.md, then compare them with the linked React Native and WinUI contribution references. Done should mean the selected contribution-guide content is repo-specific, duplicates are resolved, the building article is discoverable, and the Playground README links are corrected; the RNTester documentation remains tied to its proposed changes.