docs: add root level documentation for all libraries

[Nerixyz]

What's this all about?

This PR adds crate-/root-level documentation for all library crates. All crates except windows-implement and windows-interface, import the documentation (with doctests) from their README.md.

windows-sys and windows include examples with some required features. Rustdoc can't infer them, so the inclusion of the README is only done if they're enabled. For windows this also checks if docsrs isn't set (although docs.rs should build without any enabled features).

Doctests are running in CI with cargo test --doc -p <crate> and documentation is generated with cargo doc --no-deps -p <crate>.

You can verify the output with

cargo doc --no-deps `
  -p windows-bindgen `
  -p windows-core `
  -p cppwinrt `
  -p windows-implement `
  -p windows-interface `
  -p windows-metadata `
  -p windows-registry `
  -p windows-result `
  -p windows-strings `
  -p windows-targets `
  -p windows-version `
  -p windows `
    -F Data_Xml_Dom `
    -F Win32_Security `
    -F Win32_System_Threading `
    -F Win32_UI_WindowsAndMessaging `
  -p windows-sys `
    -F Win32_Security `
    -F Win32_System_Threading `
    -F Win32_UI_WindowsAndMessaging `
  --open

Fixes: #3187

[kennykerr]

Thanks!

[kennykerr]

windows-sys and windows include examples with some required features. Rustdoc can't infer them, so the inclusion of the README is only done if they're enabled.

Just playing around with it and it looks good for most crates but this point for the windows and windows-sys crates seems problematic as I don't see how this would ever be the case in practice so I'm wondering what it's even for. Is it just to test the readme? test_readme already does that.

[Nerixyz]

Just playing around with it and it looks good for most crates but this point for the windows and windows-sys crates seems problematic as I don't see how this would ever be the case in practice so I'm wondering what it's even for. Is it just to test the readme? test_readme already does that.

Oh, I didn't see that test. Then the doctests I added to CI can be removed (since there aren't any doctests outside the readmes).