Add links to GitHub downloads

[Lastique]

It would be nice to add GitHub download links in release notes starting with Boost 1.81.0.

For example, on 1.81.0 release notes page, add a text like "Source packages with git layout are available on GitHub." below the downloads table in the Downloads section.

The archives on GitHub are useful for people using Boost in CMake. They lack the unified boost include directory and require the user to either recreate it by running b2 headers or specify library include directories in the compiler command line (which is what CMake does).