[boost] Release Notes: Markdown to replace QBK

We need to re-render all of this historical release notes, such as: https://www.boost.org/users/history/version_1_81_0.html to be in the style of the new website. This is being taken care of, but it will affect the release process as follows: * Each author will edit the release notes for their library using markdown instead of Quickbook format, as a pull request against a file here: https://github.com/boostorg/website/tree/master/feed/history The syntax will be GitHub flavored markdown, with a few Boost-specific extensions (for example, to link against an issue in a boostorg repo). Does anyone object or have comments about this? -- Regards, Vinnie Follow me on GitHub: https://github.com/vinniefalco