Move operators precedence to main documentation

[shorodilov]

Description

Operator precedence should be explained in main documentation.
Appendix should include the precedence table and provide brief additional information.

References

https://github.com/edu-python-course/edu-python-course.github.io/blob/d7e818b35c0a4b46fe9aaa530bc4ebd87e1db0ab/src/appx/precedence.txt#L7