Add build troubleshooting suggestion (#36178)

* Add troubleshooting suggestion

Add a note about deleting SDK and tools locally if restore.cmd fails.

* Move content to BuildErrors.md

Move the content to a catch-all "Miscellaneous other errors" section.

* Update docs/BuildErrors.md

Co-authored-by: Safia Abdalla <[email protected]>

docs/BuildErrors.md

@@ -98,3 +98,9 @@ or
 ```bash
 ./build.sh --projects "$PWD/src/Framework/App.Runtime/src/Microsoft.AspNetCore.App.Runtime.csproj"
 ```
+
+## Errors when restoring older clones
+
+If you have build errors trying to run `restore.cmd` and you cloned the repository some time ago,
+try deleting the `.dotnet` and `.tools` directories in your local repo directory. It may resolve
+the problem if older versions of the .NET SDK are causing an incompatibility with the latest version.