docs: document unified memory env configuration - #1051

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="docs/unified-memory.md" line_range="20" />
<code_context>
+## docker model run
+
+```shell
+docker model run -e GGML_CUDA_ENABLE_UNIFIED_MEMORY=1 ai/gemma3 "Hello"
+```
+
</code_context>
<issue_to_address>
**issue (bug_risk):** The documented `docker model run -e GGML_CUDA_ENABLE_UNIFIED_MEMORY=1 ...` command fails because this repository's `run` command does not define an `-e`/`--env` flag, so Cobra rejects the option instead of configuring the runner.

**Triggers:** When users follow the Docker Model Runner CLI example.

**Suggested fix:** Set the variable in the environment before invoking `docker model run`, or document a supported runner/container configuration mechanism instead of passing `-e` to `docker model run`.

```suggestion
GGML_CUDA_ENABLE_UNIFIED_MEMORY=1 docker model run ai/gemma3 "Hello"
```
</issue_to_address>

### Comment 2
<location path="docs/unified-memory.md" line_range="1" />
<code_context>
+# Unified memory configuration for integrated GPUs
+
+Docker Model Runner uses llama.cpp under the hood. On systems with integrated GPUs (for example AMD APUs), available shared memory may be reported incorrectly unless unified memory is enabled.
</code_context>
<issue_to_address>
**nitpick:** The new unified-memory document is not linked from `README.md` or any existing documentation index, so the repository's stated README-link verification is false and users browsing the README cannot discover this configuration guide.

**Suggested fix:** Add a README link to `docs/unified-memory.md`, preferably alongside the existing documentation resources.
</issue_to_address>