Release Notes - legacy import

Opening this issue to not lose track of it. This could be done later, as a Priority 3 or 4 item.

- All previous release notes are available in https://github.com/boostorg/website. That repo won't be deleted any time soon.
- The code in boost.io knows how to import release notes from boostorg/website.

Therefore based on the above points, "nothing" needs to be done about previous release notes. They are already supported, and could be re-imported if necessary.

Going forward, with 1.86.0 and later, use the new ascidoc method.

Is there any reason to migrate the previous boostorg/website quickbook files to adoc format, and check them into the new website? Yes, there is a minor reason. So that someday the repo boostorg/website could be deleted, moved, renamed, etc. and no dependency on it would remain.

Tasks:
- Convert the quickbook files to adoc format
- Check them into website-v2-docs, in the same location as other release notes.
- Modify python code, to discover those files in website-v2-docs

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Edit: An alternative solution would be to "import" the html formatted versions of the release notes into https://github.com/boostorg/website-v2-docs, instead of the quickbook versions. Both quickbook and html copies are available in https://github.com/boostorg/website.