Split library usage out of the root README into `src/Temporalio/README.md`

[jmaeagle99]

What are you really trying to do?

Two unrelated audiences read this file. Someone evaluating or using the package wants installation
and usage. Someone contributing to the repository wants to know how to build the bridge, run the
tests, and regenerate code. Both are currently served by one 1,434-line document, and each has to
scroll past the other's material.

Describe the problem

The root README.md is not just the repository landing page — it is the NuGet package page.
src/Temporalio/Temporalio.csproj packs it directly:

<None Include="../../README.md" Pack="true" PackagePath="\" />

So the published page for Temporalio ends with a ## Development section explaining how to
build the Rust bridge, run the test suite, regenerate protobuf types, and configure formatting in
VS Code. None of that is actionable for someone who has installed the package.

In the other direction, a would-be contributor opening the repository has to scroll past 1,317 lines of usage
documentation before reaching contributing instructions at line 1318.

Every other package in this repository already does the thing being proposed here — each has its
own README.md next to its project and packs it locally:

src/Temporalio.Extensions.Aws.Lambda/README.md                 157 lines
src/Temporalio.Extensions.Aws.Lambda.OpenTelemetry/README.md    71 lines
src/Temporalio.Extensions.DiagnosticSource/README.md            47 lines
src/Temporalio.Extensions.Gcp.CloudRun.OpenTelemetry/README.md 117 lines
src/Temporalio.Extensions.Hosting/README.md                    197 lines
src/Temporalio.Extensions.OpenTelemetry/README.md              243 lines

Temporalio is the only package that reaches up to the repository root for its readme.

Proposed change

• Move the library content — the header, Quick Start, and Usage sections, lines 1 to 1317 — to
  src/Temporalio/README.md.
• Change Temporalio.csproj to pack README.md from its own directory, matching the six
  extension projects.
• Leave the root README.md as a repository overview: what this repo contains, where the packages
  are, the ## Development section that is already there, and a prominent link to the library
  readme.