Add PHP code optimization guide with opcache performance improvements

[Copilot]

This PR adds documentation about code-level optimizations for PHP applications, with a focus on opcache performance improvements using fully qualified function names with leading backslashes.

What was added

Added a new "Optimizing your code" section to the performance tuning guide that covers code-level optimizations separate from PHP environment configuration. The section explains:

• The concept: When calling global functions from within a namespace (e.g., count(), array_merge()), PHP first searches in the current namespace before falling back to the global namespace
• The optimization: Adding a leading backslash (\count() instead of count()) tells PHP to directly use the global function, avoiding the namespace lookup
• The benefit: This improves opcache efficiency by eliminating unnecessary namespace resolution steps
• Implementation: Recommends using automated tools like PHP-CS-Fixer with the native_function_invocation rule rather than manual implementation

Example

// Instead of this (requires namespace lookup):
namespace MyApp\Service;
$count = count($items);

// Use this (direct global function call):
namespace MyApp\Service;  
$count = \count($items);

This follows the preference expressed in the issue comments that such optimizations should be automated rather than manually maintained. The documentation properly separates environment-level PHP optimizations from code-level optimizations, maintaining consistency with the existing guide structure and formatting.

Fixes #51

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[samdark]

@copilot revert these changes.

[Copilot]

Reverted the .gitignore changes in commit 4339289.