Add copilot instructions

.github/copilot-instructions.md

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+# Development instructions
+- This is a personal project that will only ever be used by me. No public distribution is planned.
+- This is a template (this is what Dokuwiki calls themes/styles) for my personal Dokuwiki instance.
+- The project is written in PHP and targets Dokuwiki.
+- Follow general PHP best practices.
+- Follow Dokuwiki coding conventions: https://www.dokuwiki.org/devel
+- Do not use `phpunit` There are missing dependencies that make it fail.
+- Use `php -l <file>` to check for syntax errors.
+- Consider The official documentation for writing dokuwiki plugins: https://www.dokuwiki.org/devel:plugins
+- Whenever necessary, update the README file to reflect new features or changes.
+
+
+# General instructions
+- This template is intended to be used by me for many years to come
+- That means maintainability is more important than cutting edge technologies
+- Write code that is easy to understand and modify
+- Favor stability over performance unless performance is a clear requirement
+- Favor simplicity over cleverness
+- Favor explicitness over implicitness
+- Favor well-known solutions over new or exotic solutions
+- Favor documented solutions over undocumented solutions
+- Favor built-in solutions over external dependencies