Added testing of embedded PHP snippets

[mnocon]

This PR makes it possible to run code quality checks (PHPStan, CS-Fixer, Rector, Deptrac) on embedded PHP code samples.

Thanks to this, we no longer need to extract the code samples to separate files - they can stay within the Markdown files, making it easier to work with them, while keeping the advantage of the code quality tools.

How it works

Before a code quality tool is run, the code samples are extracted from Markdown files.

They are not extracted if:

• the code sample uses one of the include_file/include_code functions
• the code sample is marked with {skip-validation}

The rest of them are extracted to code_samples/_inline_php for the tools to pick up.

When the tool can automatically fix the issue (CS-Fixer, Rector), the changes are synced back to the Markdown files.

All the existing inline code samples are marked with {skip-validation}, a special attribute to disable the code sample checking.

This keeps this PR small and allow us using this for new content - the fixing of code samples will be done in a follow-up (#3206)

[github-actions]

Preview of modified files: Too many files modified in a single PR, preview link list is skipped. (309 files > 100)

[sonarqubecloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud