[BUGFIX] Validate guides.xml against XSD schema before loading

[CybotTM]

Summary

This adds proper XSD schema validation before the configuration is loaded into Symfony's container. When validation fails, users now see precise error messages with line and column numbers instead of cryptic PHP fatal errors.

Before:

PHP Fatal error:  Uncaught Symfony\Component\Config\Definition\Exception\InvalidTypeException: 
Invalid type for path "guides.theme". Expected "scalar", but got "array". in ...
Stack trace:
#0 ...
(20+ lines of stack trace)

After:

 Invalid guides.xml configuration 

Your guides.xml file failed XSD schema validation:

Schema validation failed for Documentation/guides.xml
  Line 3, Column 0: Element '{https://www.phpdoc.org/guides}theme': This element is not expected.

See: https://docs.typo3.org/m/typo3/docs-how-to-document/main/en-us/GeneralConventions/GuideXml.html

Approach

This is a comprehensive fix that:

• Validates all guides.xml files against the XSD schema upfront
• Shows exact file path, line number, and column for each error
• Links to official documentation for correct format
• Keeps a fallback catch for any edge cases not covered by XSD

Alternative

See also the companion PR with a simpler fix that only catches exceptions without XSD validation: #1117

Test plan

• Create a malformed guides.xml with <theme name="typo3docs" /> as child element
• Run bin/guides run --config=Documentation Documentation
• Verify error message shows exact line number and XSD error
• Verify valid guides.xml files still render correctly

[linawolf]

With #1117 we merged the "small variant" of this change, let us see if that is sufficient or we want to merge this, improved variant