TODO(kashida) in core/templates/expressions/expression-evaluator.service.ts: Split the section into two.

[FaithAbiola]

The purpose of the TODO comment is to split the "Defining new operators and functions" section into two parts:

1. A general overview of operators (including some concrete examples)
2. A numbered sequence of steps for a new contributor to follow in order to define a new operator

Split the "Defining new operators and functions" section into two parts as described above.
In the first part, provide a general overview of operators, including some concrete examples.
In the second part, provide a numbered sequence of steps that a new contributor should follow to define a new operator.

Ensure the content is clear and comprehensive, so that a new contributor can easily understand how to define a new operator.

[seanlip]

Closing as duplicate of #20339.