docs: Update "Adding a New Expression" to reference SQL file tests

### What is the problem the feature request solves?

We've had a number of recent PRs adding new expressions (which is phenomenal!) that I have to ask to add/migrate tests to the new SQL file test framework (https://datafusion.apache.org/comet/contributor-guide/sql-file-tests.html).

### Describe the potential solution

Add a link to SQL file tests https://datafusion.apache.org/comet/contributor-guide/sql-file-tests.html on the "Adding a New Expression" page and mention it in how to test.

https://datafusion.apache.org/comet/contributor-guide/adding_a_new_expression.html

### Additional context

_No response_