📝 Improve docs (#402)

- Better orders of each section.
- More specific features indication.
- Link more docs to PM.
- Remove redundant descriptions.
- Add descriptions about fields in `AssetPickerConfig`.
- Add pseudo codes for uploading `AssetEntity`.

Author: AlexV525

.all-contributorsrc

@@ -189,7 +189,7 @@
       "login": "hellohejinyu",
       "name": "何锦余",
       "avatar_url": "https://avatars.githubusercontent.com/u/8766034?v=4",
-      "profile": "http://blog.1qa.link",
+      "profile": "https://github.com/hellohejinyu",
       "contributions": [
         "bug"
       ]