Add Per-Instance Overrides page, update FAQ and sidebar
Adds Per-Instance Overrides wiki page covering: what it is and who
it is for, how to enable it, all seven fields explained, override
logic (sparse storage, inheritance), Sonarr vs Radarr differences,
Sample Mode and Use Global, desktop and mobile usage, reset
behaviour, and Notifications Enabled inheritance.
Updates FAQ with an entry clarifying that overrides require Apply
before they take effect.
Updates _Sidebar.md to insert Per-Instance Overrides between
Settings Reference and Radarr & Sonarr Backlog.