Restore --delete to the docs sync

[kcmartin]

Closes the second half of superfly/docs-tracking#350.

#5287 left --delete out of the rsync because the first sync would also have removed 12 pages that docs.json nav entries and 15 redirects still pointed at. Both halves have now landed: superfly/docs#2530 retired the litefs-cloud command pages, superfly/docs#2535 the duplicate root page.

Verified before restoring: generating from current master and comparing against superfly/docs main gives identical page sets, 305 each, and zero changed pages. So this deletes nothing today.

What it prevents: the litefs-cloud commands were removed in #5187 on 8 September and their eleven pages stayed live for four weeks, documenting a CLI that was not in the binary. Without --delete that happens every time a command goes.

One consequence, noted in the comment: nothing hand-written can live in flyctl/cmd now, since the sync will remove it.

[kcmartin]

Added a floor check before the rsync.

Neither this script nor publish_docs.sh uses set -e, and out/ is cleared before the generator runs. So a generator that fails leaves out/ empty, and the rsync with --delete would propagate that and remove every page. It now refuses below 100 files; there are 305 today.

Used a floor rather than set -e because set -e would abort the label-preservation loop: [ -n "$label" ] && awk ... returns non-zero for the 270 pages that carry no sidebarTitle.

Worth noting the interaction: --delete is safe partly because the generated PR now waits for a person, so a 305-deletion PR would never be merged. If we go back to auto-merge, this floor check is the only thing standing between a failed build and an empty command reference.