Some of the links in the CONTRIBUTING.md file are broken and link to non-existing pages.

Among these are links to:

• https://github.com/dotnet/maui/blob/main/CODE_OF_CONDUCT.md
• https://github.com/dotnet/maui/blob/main/PULL_REQUEST_TEMPLATE.md
• https://github.com/dotnet/maui/blob/src/Core/tests/Benchmarks

It looks like the markdown files have been moved to .github/ and the links were not updated or relative links in this folder don't work.

The benchmark project also seems to have moved or does not exist anymore.