docs: document configurable software license and help page links (theme.json common.urls)

#12528 (merged) added two optional fields to the theme's `common.urls` config in `theme.json`: `softwareLicense` and `helpPage`.

When either is set:
- A new "Help" menu button appears in the web UI topbar (`HelpMenu.vue`), shown only when at least one URL is configured.
- The account page gains additional info rows linking out to the configured URL(s).
- Both are also registered as app-menu extensions via a new `buildStaticAppMenuItems` helper, so they appear in the app switcher too.

When neither is set, behavior is unchanged (no Help button, no extra links).

**What needs updating:** the admin-facing theming/branding documentation describing `theme.json` options should gain `common.urls.softwareLicense` and `common.urls.helpPage` entries.

Draft entry:
> `common.urls.softwareLicense` (optional, string): a URL to the deployment's software license terms. When set, surfaced via the topbar Help menu, the account page, and the app switcher.
> `common.urls.helpPage` (optional, string): a URL to the deployment's help/support documentation. Same surfacing as above.

As with `logo.href` (#12536), these are `theme.json` options rather than auto-generated env vars, so this needs a manual doc update — and per that issue's investigation, the admin theming docs may live outside this repo; confirm the correct docs repo/path before editing.

🤖 Automated by Claude Code