docs: document configurable logo click-through URL (theme.json `logo.href`)

#12529 (merged) added an optional `href` field to the theme's logo config in `theme.json`. When set, clicking the topbar logo navigates to that URL instead of the default internal navigation; when unset, behavior is unchanged.

**What needs updating:** the admin-facing theming/branding documentation (the page(s) describing `theme.json` logo options such as `logo.topbar`/`logo.login`/`logo.favicon`) should gain a `logo.href` entry.

Draft entry:
> `logo.href` (optional, string): a URL. When set, clicking the topbar logo navigates to this URL instead of the app's default home route. When unset, the existing internal-navigation behavior is preserved.

Unlike `services/*/pkg/config` env vars, `theme.json` options aren't covered by the auto-generated env-var docs, so this needs a manual doc update.