Analyze code samples in documentation with PHPStan (#325)

* Analyze code samples in documentation with PHPStan

* Replace colons with semicolons in readme samples

* Add missing parentheses

* Remove "public" visibility from local functions in readme samples

* Fix invalid code examples in README

* Add return type to functions in readme examples

* Ignore resultUnused errors in doc code snippets

* Fix all remaining PHPStan issues in README samples

* Fix phpstan.neon indenting and override analyzed paths for docCodeAnalyzer

* Update dependency to doc code analyzer package to version that doesn't have fallback paths configured

Author: PrinsFrank