HeadDB documentation update

Updated against the HeadDB 7.0.0-rc.6 project state.

Content changes

Replaced public-facing “More Heads” and “More Categories” terminology with “Custom Heads” and “Custom Categories”.
Documented legacy more-heads and more-categories permission identifiers as compatibility names rather than current feature names.
Updated the command reference, aliases, permissions, /hdb report, search subcommands, custom taxonomy commands, and GUI-first edit workflow.
Updated the complete permissions reference, including command, GUI, metadata-editing, category, and .others permissions.
Added sounds.yml to installation, configuration, backup, troubleshooting, and GUI documentation.
Added scheduled database refresh configuration and behavior, including the default six-hour interval and unchanged-artifact optimization.
Clarified LOADED from CACHE, startup refresh, manual verification, manual refresh, and scheduled checks.
Updated updater documentation to distinguish database updates from plugin JAR updates.
Updated Custom Head and Custom Category drafts, publishing, persistence, and Browse integration.
Updated economy terminology and configuration examples.
Updated the API dependency version to 7.0.0-rc.6 and added Bukkit service lookup examples.
Updated troubleshooting and FAQ pages for current behavior.
Updated the home page positioning and feature descriptions.