docs: Add `go install` as a first-class installation method

> [!Note]
> Responses generated with Claude

### Problem

The README currently documents Docker/container as the primary installation method, with "Build from source" (`go build`) as a fallback framed as "If you don't have Docker." There's no mention of `go install`, which is the standard one-liner for installing Go binaries.

### Proposed change

Add `go install` alongside the container option as a first-class installation method:

```bash
go install github.com/github/github-mcp-server/cmd/github-mcp-server@latest
```

Then configure the MCP client to use the local binary:

```json
{
"mcpServers": {
"github": {
"command": "github-mcp-server",
"args": ["stdio"],
"env": {
"GITHUB_PERSONAL_ACCESS_TOKEN": ""
}
}
}
}
```

### Why

- **One command, no clone needed** — `go install` fetches, builds, and places the binary in `$GOPATH/bin` in one step
- **Faster startup** — no container overhead on each MCP server launch
- **No Docker/Podman dependency** — many developers have Go installed but not a container runtime
- **Lower resource usage** — containerized MCP servers consume memory even when idle, limiting the number of IDEs or agent terminals that can run concurrently. A native binary uses no resources when not actively handling a request.
- **Safe** — the binary only makes outbound HTTP calls to the GitHub API via stdio; container isolation adds no meaningful security benefit here
- **Standard Go practice** — `go install` is the idiomatic way to distribute Go CLI tools

### Verified

Tested on macOS (darwin/arm64):
```
$ go install github.com/github/github-mcp-server/cmd/github-mcp-server@latest
go: downloading github.com/github/github-mcp-server v1.0.1
$ github-mcp-server --version
GitHub MCP Server - Version: version
```