Add links to GitHub downloads

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](https://www.boost.org/users/history/version_1_81_0.html) page, add a text like "Source packages with git layout are available on [GitHub](https://github.com/boostorg/boost/releases/tag/boost-1.81.0)." 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).