bisync: replace the docs changelog with a link to its archived copy

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Before this change, `docs/content/bisync.md` kept its own changelog, and
each bisync pull request added a line under the same heading. The rclone
changelog is already made from commit messages (`bin/make_changelog.py`),
so the entries duplicated it, and caused unnecessary merge conflicts: open
pull requests conflicted with each other whenever one of them merged.

After this change, the section links to the rclone changelog and to the
last version of the bisync changelog on GitHub. The `## Changelog` heading
stays, so links to `/bisync/#changelog` still work.

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