Diagnostics repo docs update

Our README.md and few other places have some missing documentation. For example, the README is strangely focused on SOS (probably because the repo started out as a repo for out-of-band SOS). It also doesn't mention anything about Microsoft.Diagnostics.NETCore.Client.

This issue tracks documenting the repo itself so that it makes it easier for those outside the .NET diagnostics team can more easily onboard/contribute to the repo.

Update README.md to better reflect the current status of the repo
- [ ] Add more descriptions about the tools
- [ ] Add links to official docs.microsoft.com landing page on diagnostics

Include onboarding docs (i.e. workflows) and CONTRIBUTING section to README.md or under docs/
- [ ] Building and Testing (we already have some, but they're targeting SOS only) and doesn't cover all platforms (i.e. only Windows has something about how to test it)
- [ ] Dogfooding - For the tools, it might be helpful to include instructions on how to build the tools and use the package to install and use them locally

If anyone else can think of other things to document in this area, please feel free to add.