This changes the doc-sync meson target from a simple rsync command to a
script that:
* puts the documentation in a subdirectory according to the version
* injects a bit of javascript to add a drop-down to switch between versions
* updates an index.json file with the newly uploaded version
* keeps the latest/ directory up to date with the latest version
* supports a --no-latest switch to be used when uploading older versions
3c1f396f69