docs: order the site navigation by how a developer adopts the library

[allen0099]

Reorders the documentation navigation (English and zh-TW) and the Documentation list in the README and the zh-TW home page to follow the order in which a developer typically adopts the library. No page content or URL changes, so existing links keep working.

New order

• Home
• Migrating to 0.4.0: moved up from after the guides. 0.3.9 is the last 0.3.x and warns about the 0.4.0 changes, so this is the first page an upgrading user looks for.
• Caching: HTTP caching → Application cache → Backends → Distributed lock. The first two pages work on the default in-memory backend. Backends is where you go when you deploy more than one process. CacheLock comes after it because it needs a shared backend and builds on the atomic primitives that page describes.
• Sessions and sign-in: Session management → OAuth state → JWT claims. This is a separate group, so readers who only cache can stop after the first. JWT claims is an extension guide and comes last.
• Under the hood: Cache flow. The internals of a cached request, moved out of the guides. HTTP caching still links to it.
• API reference: reordered to match the guides (HTTP caching, CacheManager, Backends, CacheLock, Session, State, Types and exceptions).
• Development, Changelog: unchanged.

The English nav label "State management" becomes "OAuth state", matching the page's subject, the README and the zh-TW nav. The README and zh-TW home Documentation lists now link to the migration guide first.