Prepare for publishing to Package Manager

[yczhangsjtu]

1. Reorganize Menu Items: Move everything into Window/UIWidgets

2. Delete Documentation~: The documents will be generated by script.

3. Prepare missing .sample.json

4. Create bash script scripts/packman: currently what it does is

1. If invoked with no parameter, clone [email protected]:unity/com.unity.uiwidgets.git to temporary dir and set as the target repository
2. If invoked with a parameter, use that as the target repository and check it's validity
3. Copy the UIWidgets repo to a temporary directory, cd to it, reset to target version
4. Copy specific files and directories to the target directory
5. Generate the documentation files from the README.md, README-ZH.md, CONTRIBUTING.
6. cd to target directory and commit