docs(config): clarify skeleton and template directory behavior

[joshtrichards]

Summary

Clarify the skeletondirectory and templatedirectory docs in config.sample.php.

This tries to make clearer:

• each options purpose
• when copying/initialization occurs
• language fallback behavior
• default values
• the independence of the template source path
• the distinction between copying skeleton or template content and configuring a user's template directory.

This should reduce ambiguity around how these settings interact (I hope!).

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI