Explore including changelog in docs

We don't have a revision history section (changelog) in the documentation. We have been counting on boost release notes only. When looking at a specific lib it's not particularly convenient to have to look in the boost release notes for its changes.

We can move that content to the docs and start from there. Then we can maintain it in my docs and copy it to the boost release notes.

Start by reviewing the current Boost.URL documentation and the Boost release notes to identify how library-specific changes are recorded. Define a documentation revision-history section that can be maintained with the library docs and copied to the release notes; the issue does not name specific files or tests.

Written by the indexing model from the issue text.