Rename solution, readme and icon files

- Rename NuGet Icon to `Icon.png`
  - This is no longer used as a public reference point for NuGet icon URL. Hence, it's safe to change.

- Normalize casing for `ReadMe.md`
  - To match the file already in Windows toolkit repo.

- Rename solution file to `CommunityToolkit.sln`
  - The `dotnet` seems implied and also doesn't stand-out in the file list because of the lower casing and `d` char.
  - Spaces are a main issue when doing automation (_like using `*.sln` in build scripts and in URLs it adds `%20`_).

Author: Nirmal4G

dotnet Community Toolkit.sln renamed to CommunityToolkit.sln

@@ -30,7 +30,7 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Docs", "Docs", "{88C6FFBE-3
 		CODE_OF_CONDUCT.md = CODE_OF_CONDUCT.md
 		Contributing.md = Contributing.md
 		License.md = License.md
-		README.md = README.md
+		ReadMe.md = ReadMe.md
 		ThirdPartyNotices.txt = ThirdPartyNotices.txt
 	EndProjectSection
 EndProject