feat: add config header and tests.

[Keeper-of-the-Keys]

• Resolves: [Bug]: Upgrade process re-renders config.php in a less readable/useable fashion #55171 (implements part of the suggested fix)
• Second attempt after feat: add config header #58886

Summary

config.php is re-rendered by nextcloud regularly removing comments and rendering constants into their values, nextcloud supports using multiple config files and renders their content into the main config.php without modifying the external files.
This PR adds a header to config.php warning the admin/user of this and suggesting the use of the multiple config files mechanism.

TODO

• In the future adding a footer that lists the files that were rendered into config.php in the order that they were included will probably help admins troubleshoot if a config variable is doubled up by mistake.
• Expand documentation more at https://docs.nextcloud.com/server/33/admin_manual/configuration_server/config_sample_php_parameters.html#multiple-merged-configuration-files with examples
• Add example/suggestion to use separate file for DB configuration https://docs.nextcloud.com/server/33/admin_manual/configuration_database/linux_database_configuration.html#ssl-for-mysql-database

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

[Keeper-of-the-Keys]

Tagging: @provokateurin @miaulalala