Introduce secondary site nav, relocate nav items

[huntie]

Note

Stacked PR: Depends on #5186.

Summary

Introduces a secondary navbar above every docs-style section of the website, and renames the top-level "Development" menu item to "Docs".

Motivation

This, IMO, fixes a missing part of the site navigation story as reactnative.dev has evolved over time.

Principally, the navbar heading helps reinforce where you are on the site (motivated by the incoming new "Releases" section) — but also improves general context and navigation across the site.

Before	After
Docs > APIs
	Docs > APIs breadcrumb/selector at page level, section-specific version dropdown now in secondary nav, copy button migrated
Contributing
Blog index
	(No copy button on index page)
Blog single
	Improved signalling via "All posts" back button, copy button now available

(Home, Showcase, etc pages are unchanged.)

This also significantly improves navigation UX on mobile:

What moved where

• Section dropdown: Guides / Components / APIs / Architecture now live in a dropdown, shown both in the top navbar "Docs" entry and in the secondary nav; the active section follows the current doc's sidebar.
• Version dropdown: Relocated from the top navbar into the secondary nav (main docs only) as a pill — brand blue for "Next", otherwise the same outlined style as the section dropdown.
• Copy page widget: Moved out of the article body into the secondary nav, on docs, architecture, community and contributing pages and on individual blog posts (not the blog index).

Other changes

• Each section shows its name (Docs / Community / Contributing / Blog) in the secondary nav, above an inset bottom keyline.
• On the blog, an "All posts" back link sits beside the heading on every route except the listing.
• Dropped the now-redundant "Community" sidebar category and the "All Blog Posts" blog sidebar heading.

🚧 Open questions

• The top level "Docs" nav item retains its dropdown of 4 options ("Guides", "Components", ...) — technically a repeat of what the secondary nav offers. This could be dropped entirely, in favour of "single click to go to docs" (reduce overhead at this point).
  • Decision: For now, keep both (do not regress ability to navigate fast from Homepage → Docs subsection. Could revisit this in future.

Test Plan

Comparison tour — desktop

Screen.Recording.2026-07-25.at.16.59.32.mov