docs(dev): add copyable PHP-CS-Fixer and ESLint setup - #15711

Summary

Coding standards pages linked to nextcloud/coding-standard and @nextcloud/eslint-config but had no copyable setup steps, and npm.rst still showed the legacy eslint --ext CLI.

• php.rst: add composer require --dev nextcloud/coding-standard, a minimal .php-cs-fixer.dist.php, and cs:check / cs:fix scripts.
• javascript.rst: add install + flat eslint.config.js using @nextcloud/eslint-config recommended.
• npm.rst: replace eslint --ext with directory-based eslint src for flat config.

Closes #15684

Test plan

• Sphinx build of the developer manual succeeds for php / javascript / npm pages
• php.rst Setup lists composer require, .php-cs-fixer.dist.php, and cs:check / cs:fix
• javascript.rst shows eslint.config.js importing recommended from @nextcloud/eslint-config
• npm.rst lint scripts no longer use --ext

AI disclosure

This PR was drafted with the help of AI coding assistants (Cursor agents / LLM-based tools), including the description. I am responsible for it and happy to rework anything that doesn't fit.

All commits carry an Assisted-by: Cursor:grok-4.7 trailer.