Added more details to Operator Precedence section
From Issue #14933.
- Made Operator Precedence a higher heading, since it's not really a subsection of `##Numeric Comparisons`, and it references all the operators described in earlier sections on the page.
- Added a link to the source file that lists *all* operator precedences.
- Added code examples for `Base.operator_precedence` to show how to programmatically find an operator's precedence.
