docs: restructure the migration guide around topical groups with a navigation layer

The v1-to-v2 migration guide had grown by accretion: one section per
breaking PR, in merge order, under four coarse buckets. Restructure it
so both a skimming human and a searching agent can find what applies
to them:

- Add a navigation layer: a symptom-keyed table of the changes almost
  every project hits, an area index, and a suggested migration order.
- Group sections topically (packaging, types/wire, MCPServer, lowlevel
  Server, clients, transports, OAuth/server auth, validation and wire
  behavior, testing, deprecations), ordered the way a migration
  actually proceeds.
- Drop sections that describe 2026-07-28 features rather than v1-to-v2
  breaks (Mcp-Param client mirroring, cache hints, extensions API,
  identity assertion, modeled protocol fields, additive lowlevel app);
  that content lives in whats-new.md and the feature pages. The
  server-side Mcp-Param section stays, under a clearly scoped "Notes
  for 2026-era connections" heading, since whats-new.md links to it.
- Merge three satellite sections into their parents and move behavior
  changes that were filed under "Bug Fixes"/"New Features" into the
  appropriate groups.
- Add missing breaks found by a v1.28.1-vs-v2 surface audit: float
  timeouts replacing timedelta, the 408-to--32001 timeout error code,
  lowlevel tool-handler exceptions becoming JSON-RPC errors,
  streamable-HTTP non-2xx handling, the _meta envelope and
  OpenTelemetry dependencies, OAuth offline_access/prompt=consent and
  issuer validation, stricter token-endpoint client authentication,
  MCPServer constructor positional-slot changes, the default server
  name change, dependency floor bumps, and several smaller renames
  folded into existing sections.
- Fix stale or wrong claims found while validating every section
  against both runtimes: model_dump() now needs by_alias=True for the
  wire shape, Before-examples that never worked in v1 as shown, the
  StatelessModeNotSupported reference (never in any v1 release), and
  the mcp.shared.context removal claims.

Heading anchors that other pages deep-link (whats-new.md) are
unchanged. mkdocs build --strict and markdownlint pass.

Author: maxisbey