Developer manual: copyable setup for PHP-CS-Fixer and ESLint #15684

The coding standards pages link to nextcloud/coding-standard and the ESLint config, but there are no steps to copy.

• getting_started/coding_standards/php.rst: add composer require --dev nextcloud/coding-standard, a minimal .php-cs-fixer.dist.php, and cs:check / cs:fix composer scripts.
• getting_started/coding_standards/javascript.rst: add a minimal current ESLint config using @nextcloud/eslint-config.
• digging_deeper/npm.rst (around line 93) uses eslint --ext; check it against the current flat config and update.