Insert Design on the ASSEMBLE panel and in the File menu (#1240) (#1405)

component.insert_design (a saved part as a linked component) had no button, so an
assembly of separately saved parts looked impossible. It is now on the ASSEMBLE panel
of every tab and in the File menu; it asks for the file, and a STEP or mesh file picked
there is inserted as Insert STEP or Insert Mesh would.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/book/src/guide/assemblies.md

@@ -64,8 +64,10 @@ instead, so two origins given at one point with the same z stay as they are. For
 drive many steps, `quiet: true` leaves the per-part degrees of freedom out of joint answers, and
 `motion.play` with `quiet: true` gives the poses of only the parts that moved.
 
-**Insert Design** (`component.insert_design {path, translate?}`) adds a component linked to
-another saved design, so each part can live in its own document. The part's solid bodies come
+**Insert Design** (`component.insert_design {path, translate?}`, on the **ASSEMBLE** panel and
+in the **File** menu) adds a component linked to another saved design, so each part can live in
+its own document. A STEP or mesh file picked there is inserted as **Insert STEP** or **Insert
+Mesh** would. The part's solid bodies come
 in as they sit in that design, and they are kept in the assembly, so it opens even without the
 part file. The link is stored relative to the assembly's folder once the assembly is saved.
 After the part file changes, **Update Links** (`component.update_links`) reads it again.