[Docs] Add "Feature Matrix" or "Capability Status" table to Provider pages

### Is your feature request related to a problem? Please describe.
Currently, the "Supported Providers" documentation lists which drivers exist (e.g., EC2, Azure ARM, GCE), but it is difficult for a user to determine the **depth of support** for each provider without diving into the source code.

**The Problem:**
A user looking at the [Compute Providers page](https://libcloud.apache.org/supported_providers.html) sees that "Amazon EC2" is supported. However, they cannot easily verify:
1. Does it support recent features like IMDSv2 (Instance Metadata Service v2)?
2. Does it support Spot Instance requests via the unified API?
3. Are the `list_sizes` or `list_images` methods hardcoded or dynamically fetched?

This opacity makes it hard for architects to evaluate Libcloud against the native SDKs (Boto3, Azure SDK) for their specific needs.

### Describe the solution you'd like
I propose enhancing the documentation for major providers (Compute/Storage) to include a **Capability Status Matrix**.

**Proposed Documentation Update:**
On the individual provider pages (or a centralized matrix), we should track the support status of standard API methods.

*Example Table:*
| Feature | Support Status | Notes |
| :--- | :--- | :--- |
| `create_node` | ✅ Full | Supports user_data, SSH keys |
| `list_sizes` | ⚠️ Partial | Hardcoded list (last updated 2024) |
| `deploy_node` | ✅ Full | |
| `ex_start_node` | ✅ Extension | Provider-specific extension |
| `IPv6 Support` | ❌ No | |

### Describe alternatives you've considered
* **Status Quo:** Users must install the library and run `dir(driver)` or inspect the source code to find capabilities.
* **Automated Badge:** Generating this matrix automatically from the test suite (e.g., based on which methods are overridden in the driver class). This would be the ideal long-term solution.

### Additional Context
As cloud providers add features rapidly, Libcloud's value proposition is "unification." Clear documentation on *what* is unified vs. what requires extension methods (`ex_*`) is critical for adoption.

libcloud.apache.org/supported_providers.html の「Supported Providers」ページから始め、個々の Compute および Storage provider のページを確認します。マトリックスを provider のページに置くべきか、集中管理されたドキュメントに置くべきかを判断し、そのうえで関連する driver から標準メソッドと provider 固有の ex_* 拡張を一覧化します。選定した主要 provider について、対応状況と注記が文書化され、スコープとメンテナンス方針が明示されていれば完了です。

索引モデルが issue の本文から書いたものです。