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).

Start with the Boost 1.81.0 release notes page and inspect the Downloads section and the website source that generates or contains it. Add GitHub release links for Boost 1.81.0 onward below each downloads table, then verify the displayed links point to the corresponding GitHub release pages.

Written by the indexing model from the issue text.