Migrate WP SEO settings page under a new top-level WP SEO admin menu

### Description

v2.0 introduces a new top-level "WP SEO" admin menu (see #131 and docs/adr/0004-readonly-settings-page-in-v2-0.md) to hold the readonly feature registry display. For v2.0, that menu will contain only the feature registry page -- the existing content-configuration settings page (currently Settings -> SEO, registered via add_options_page() in php/class-wp-seo-settings.php) stays where it is.

This issue tracks migrating that existing settings page (Home Page, Post Types, Open Graph, per-post-type/taxonomy archive sections, Search Results, 404 Page, etc.) to live as a sibling submenu under the new top-level WP SEO menu, so plugin configuration isn't split across two disconnected admin locations (Core's Settings sidebar and the new WP SEO menu).

### Use Case

Avoid a permanently split admin surface: one place for "is this feature on" (new WP SEO menu) and another for "how is it configured" (Core Settings sidebar). Deferred out of #131's scope to avoid coupling the readonly feature display to a migration of the existing, larger settings page.