map_export now saves welded airlocks and paper on noticeboards (#91960)

When using `map-export`:
- Exports the welded state of an airlock to the welded airlock mapping
helper
- Exports the paper inside of a noticeboard to pop out and be saved on
the same turf

Both `airlocks` and `noticeboards` handle the outside objects already in
their `Initialization()` procs so I didn't have to change anything. Also
I cleaned up the paper code and added some better code documentation to
`on_object_saved()`.

We convert this:

![dreamseeker_h4gQeOTaSC](https://github.com/user-attachments/assets/356b976f-b597-4492-96b0-7169b22e1845)

To this:

![StrongDMM_lQ5B1i8lH5](https://github.com/user-attachments/assets/b5360967-3e36-4e32-b94d-a51245eab867)

## Why It's Good For The Game
There weren't any good examples of using `on_object_saved()` and the
documentation was lacking so I wanted to include some proof of concept
for a few objects. Also airlocks already save their welded state, but I
think it looks better to have it as a mapping helper since it shows up
in StrongDMM instead of being a VV edit.

code/game/objects/structures/noticeboard.dm

@@ -13,18 +13,27 @@
 
 MAPPING_DIRECTIONAL_HELPERS(/obj/structure/noticeboard, 32)
 
+/obj/structure/noticeboard/on_object_saved()
+	var/data
+
+	for(var/obj/item/paper/paper in contents)
+		var/metadata = generate_tgm_metadata(paper)
+		data += "[data ? ",\n" : ""][paper.type][metadata]"
+
+	return data
+
 /obj/structure/noticeboard/Initialize(mapload)
 	. = ..()
 
 	if(!mapload)
 		return
 
-	for(var/obj/item/I in loc)
+	for(var/obj/item/paper/paper in loc)
 		if(notices >= MAX_NOTICES)
 			break
-		if(istype(I, /obj/item/paper))
-			I.forceMove(src)
-			notices++
+
+		paper.forceMove(src)
+		notices++
 	update_appearance(UPDATE_ICON)
 	find_and_hang_on_wall()