docs: add Podman compatibility warning (#12120)

## Summary
Adds a warning to the Getting Started docs clarifying that **Podman and
podman-compose are not supported**.

## Problem
Users on Windows using `podman-compose` instead of Docker get errors
like:
```
Error: the specified Containerfile or Dockerfile does not exist, ..\..\autogpt_platform\backend\Dockerfile
```

This is because Podman handles relative paths differently than Docker,
causing incorrect path resolution on Windows.

## Solution
- Added a clear warning section after the Windows WSL 2 notes
- Explains the error users might see
- Directs them to install Docker Desktop instead

Closes #11358

<!-- greptile_comment -->

<details><summary><h3>Greptile Summary</h3></summary>

Adds a "Podman Not Supported" warning section to the Getting Started
documentation, placed after the Windows/WSL 2 installation notes. The
section clarifies that Docker is required, shows the typical error
message users encounter when using Podman, and directs them to install
Docker Desktop instead. This addresses issue #11358 where Windows users
using `podman-compose` hit path resolution errors.

- Adds `### ⚠️ Podman Not Supported` section under Manual Setup, after
Windows Installation Note
- Includes the specific error message users see with Podman for easy
identification
- Links to Docker Desktop installation docs as the recommended solution
- Formatting is consistent with existing sections in the document (emoji
headings, code blocks for errors)
</details>


<details><summary><h3>Confidence Score: 5/5</h3></summary>

- This PR is safe to merge — it only adds a documentation warning
section with no code changes.
- The change is a small, well-written documentation addition that adds a
Podman compatibility warning. It touches only one markdown file,
introduces no code changes, and is consistent with the existing document
structure and style. No issues were found.
- No files require special attention.
</details>


<details><summary><h3>Flowchart</h3></summary>

```mermaid
flowchart TD
    A[User wants to run AutoGPT] --> B{Which container runtime?}
    B -->|Docker / Docker Desktop| C[docker compose up -d --build]
    C --> D[AutoGPT starts successfully]
    B -->|Podman / podman-compose| E[podman-compose up -d --build]
    E --> F[Error: Containerfile or Dockerfile does not exist]
    F --> G[New warning section directs user to install Docker Desktop]
    G --> C
```
</details>


<sub>Last reviewed commit: 23ea6bd</sub>

<!-- greptile_other_comments_section -->

<!-- /greptile_comment -->

docs/platform/getting-started.md

@@ -218,6 +218,17 @@ If you initially installed Docker with Hyper-V, you **don’t need to reinstall*
 
 For more details, refer to [Docker's official documentation](https://docs.docker.com/desktop/windows/wsl/).
 
+### ⚠️ Podman Not Supported
+
+AutoGPT requires **Docker** (Docker Desktop or Docker Engine). **Podman and podman-compose are not supported** and may cause path resolution issues, particularly on Windows.
+
+If you see errors like:
+```text
+Error: the specified Containerfile or Dockerfile does not exist, ..\..\autogpt_platform\backend\Dockerfile
+```
+
+This indicates you're using Podman instead of Docker. Please install [Docker Desktop](https://docs.docker.com/desktop/) and use `docker compose` instead of `podman-compose`.
+
 
 ## Development