docs: move docs to root dir

Before submitting a pull request, please make sure the following is done...

[ ] Ensure the pull request title and commit message follow the Commit Specific in English.
[ ] Fork the repo and create your branch from master or formily_next.
[ ] If you've added code that should be tested, add tests!
[ ] If you've changed APIs, update the documentation.
[ ] Ensure the test suite passes (npm test).
[ ] Make sure your code lints (npm run lint) - we've done our best to make sure these rules match our internal linting guidelines.

Please do not delete the above content

formilyjs / element-plus