Add GETTING_STARTED_V1.6.md beta guide

[Aaronontheweb]

Purpose

The Akka.NET website is frozen on v1.5 docs until v1.6 stabilizes. Beta users
have no in-repo guide to work from. This PR adds one: GETTING_STARTED_V1.6.md
at the repo root, rendered directly on GitHub, plus a short pointer to it near
the top of README.md.

Merge order: merge this after #8701. #8701 refreshes
docs/articles/deployment/native-aot.md, which the Native AOT section here
links to for details - on dev today that page is stale relative to the AOT
work that already landed.

Sections

1. What's in v1.6 - net10.0-only targeting, Akka.Hosting shipping from this
   repo (package ids, the version-alignment requirement, Microsoft.Extensions.*
   10.0.0 floor), Artery, the source-generated V2 serializer, and Native AOT
   support in progress. Links BREAKING_CHANGES_V1.6.md.
2. Upgrading from 1.5 - third-party plugin compatibility and a pointer to
   the ledger's Wire rows before a rolling upgrade.
3. Getting started with Akka.Hosting - the package references (including
   the Microsoft.Extensions.Hosting package Akka.Hosting itself doesn't
   carry) and a minimal Host.CreateApplicationBuilder + AddAkka sample.
4. Native AOT and trimming - what CI verifies today (a local ActorSystem
   and an Akka.Hosting app, both PublishAot), what's only been manually
   probed (Remote over Artery - classic DotNetty does not work under AOT at
   all), what's unsupported for now (Cluster, Persistence, cluster tools), the
   Setup APIs that replace HOCON type names (and their one-setup-per-type
   gotcha), and the stream-refs/manifest-serializer exceptions. Links the
   fuller docs/articles/deployment/native-aot.md for details instead of
   repeating it.
5. Source-generated serializer (Akka.Serialization.V2) - package reference,
   protocol interface, message type, serializer declaration, registration, and
   how it composes (or doesn't) with Akka.Hosting's AddSetup/WithCustomSerializer.
   Links docs/articles/serialization/source-generated-serialization.md for
   the full reference.
6. Enabling Artery - the HOCON to turn it on, the experimental/production
   warning Akka.NET logs itself, the address-scheme/port change, and the one
   supported transport.
7. Getting help / reporting beta issues - GitHub Issues/Discussions/Discord.

Verification

• All three C# samples (Akka.Hosting minimal sample, the Setup-APIs sample used
  in the Native AOT section, and the Akka.Serialization.V2 sample) were built
  and run as standalone console apps referencing the real projects in this repo
  (Akka.csproj, Akka.Hosting.csproj, Akka.Serialization.V2.csproj +
  its generator wired in as an analyzer, the same way Akka.Remote.csproj
  does). Each one compiled and produced the expected output; the code in the
  doc is an exact subset of what ran - re-verified again after a round of
  review fixes by rebuilding from the final doc text verbatim.
• The AOT csproj snippet was checked both ways: PublishAot=true alone (no
  RuntimeIdentifier condition) against a ProjectReference to Akka.csproj
  produces ~20 IL2026/IL2067/etc. trim warnings on dotnet publish; adding
  <RuntimeHostConfigurationOption Include="Akka.DynamicTypeLoading" Value="false" Trim="true" /> reduces that to zero and the published native
  binary runs correctly.
• The version-float claim was checked directly: Version="*-beta*" resolves
  to Akka 1.5.71 on dotnet restore (confirmed via project.assets.json);
  Version="1.6.0-beta*" fails with NU1102 as intended.
• builder.AddSetup(registration.CreateSetup()) from Akka.Hosting, composed
  with a source-generated Akka.Serialization.V2 serializer, was built and run
  end to end.
• Every other claim (target frameworks, package ids, the AotCanary CI job,
  the Akka.DynamicTypeLoading switch, Artery's HOCON/transport support and
  its runtime warning/address scheme, the generator's diagnostics) was checked
  against the current dev source, not assumed.
• markdownlint (same config as CI's .markdownlint.json) and cspell (CI's
  docs/cSpell.json) both run clean against the new file and the README
  change, and the full docs/**/*.md tree still passes after the dictionary
  addition. Note: CI's own lint/spellcheck jobs only glob docs/**/*.md, so
  these two root-level files aren't covered by CI - I ran both tools locally
  against them anyway and added netstandard to the shared dictionary.

Test plan

• Render GETTING_STARTED_V1.6.md on GitHub and confirm the in-page anchor
  links resolve
• Confirm the README callout link resolves
• Re-check the Native AOT section's link target once Docs: update Native AOT support scope for v1.6 #8701 lands