solid.mirror: mirror bodies of placed components where they are shown (#1896)

Thanks @NOWN-DEV.

crates/doc/src/format/pending/mirror-placements.rs

@@ -0,0 +1,5 @@
+//! A Mirror of bodies keeps the placement each body was picked at (`placements`: one occurrence
+//! path per body), and mirrors the body where that placement shows it, so a body of a moved or
+//! inserted component, or of one of its further instances, is mirrored where it is seen. Older
+//! designs need no transformation: a Mirror without `placements` mirrors the bodies' own
+//! geometry as before; the last release asks for an update instead of mirroring them there.