Update and extend recommendations on Unicode usage in Coding guidelines

[dhomeier]

What is the problem this feature will solve?

Currently our Developer docs 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 setting rather restrictive regulations for the Python standard library
• Security considerations that may become more relevant when using more easily confusable symbols.

Thoughts?

Describe the desired outcome

No response

Additional context

No response

[pllim]

Might be a good topic for Coordination Meeting 2025. 🐱 astropy/astropy-project#430 (comment)