Add comprehensive guide for writing AGLint linter rules

- Created new documentation file `docs/writing-rules.md` with detailed guide on writing custom linter rules
- Covers rule structure, metadata configuration, and the `defineRule` helper function
- Explains the context object and its properties/methods for accessing source code and AST
- Documents visitor pattern with CSS-like selectors using esquery (basic, descendant, child, attribute, wildcard, and complex selectors)
- Includes reference to AS

Author: totadavid95