[#7201] [Improvement] fix incorrect paths for playground links (#7277)

[#7201] [Improvement] fix incorrect paths for playground links

### What changes were proposed in this pull request?

This pull request fixes incorrect relative paths in a markdown link in
the file:

```

docs/getting-started/playground.md

````

The original link:

```md
[How to Use the Playground](./how-to-use-the-playground.md)
````

was incorrect because the target file resides in the parent directory.
The link was corrected to:

```md
[How to Use the Playground](../how-to-use-the-playground.md)
```

### Why are the changes needed?

The incorrect link caused documentation build warnings during CI:

```
Warning: Docs markdown link couldn't be resolved: (./how-to-use-the-playground.md)
```

Fixing this:

1. Removes the warning during the site build process.
2. Ensures that documentation links are functional and accurate.
3. Improves the developer experience and the usability of the
documentation site.

### Does this PR introduce *any* user-facing change?

No.

### How was this patch tested?

Need to check in the CI logs

---------

Co-authored-by: raveendra11 <Gurthuledu@1>

Author: raveendra11

docs/getting-started/playground.md

@@ -19,5 +19,5 @@ newgrp docker
 You can install and run all the programs as Docker containers by using the
 [gravitino-playground](https://github.com/apache/gravitino-playground).
 For details about how to run the playground, see
-[how-to-use-the-playground](./how-to-use-the-playground.md)
+[how-to-use-the-playground](../how-to-use-the-playground.md)