Update and extend recommendations on Unicode usage in Coding guidelines

### What is the problem this feature will solve?

Currently our [Developer docs](https://docs.astropy.org/en/latest/development/codeguide.html#unicode-guidelines) only have some cautionary recommendations about using Unicode characters in printed output:
> For maximum compatibility, we need to assume that writing non-ASCII characters to the console or to files will not work.

With #17651 merged there is now precedence for using non-ASCII characters in functional code, so the docs should be updated to reflect this, but also give some guidelines or rules when and how to use Unicode in code. Potentially to consider:
- [PEP 3131](https://peps.python.org/pep-3131/#policy-specification) setting rather restrictive regulations for the Python standard library
- [Security considerations](http://www.unicode.org/reports/tr36/) that may become more relevant when using more easily confusable symbols.

Thoughts?

### Describe the desired outcome

_No response_

### Additional context

_No response_

Start with the Developer docs' Unicode guidelines section linked in the issue, then review the precedent from #17651, PEP 3131, and Unicode security considerations. Clarify and document maintainable recommendations for Unicode in functional code and printed output, with the finished guidance agreed by project maintainers.

Written by the indexing model from the issue text.