Add docset generation (#24322)

(re-land of #24244)

This generates a zip file containing all of the docs, and uploads it when we publish docs, as well as a
Dash/Zeal docset that contains a feed of the docs.

Addresses at least part of #9955

dev/docs/dashing.json

@@ -1,7 +1,6 @@
 {
   "name": "flutter",
   "package": "flutter",
-  "version": "{{VERSION}}",
   "author": {
     "name": "The Flutter Team",
     "link": "https://flutter.io"
@@ -102,4 +101,4 @@
   "ignore": [
     "ABOUT"
   ]
-}
+}