[SPARK-52541][SDP] Add programming guide for Declarative Pipelines

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

Adds a programming guide for the Declarative Pipelines subproject.

### Why are the changes needed?

To teach users about Declarative Pipelines.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

Rendered the markdown and inspected it

### Was this patch authored or co-authored using generative AI tooling?

Closes #51234 from sryza/programming-guide.

Lead-authored-by: Sandy Ryza <[email protected]>
Co-authored-by: Sandy Ryza <[email protected]>
Signed-off-by: Sandy Ryza <[email protected]>

Author: sryza

docs/_layouts/global.html

@@ -80,6 +80,7 @@
                             <a class="dropdown-item" href="{{ rel_path_to_root }}graphx-programming-guide.html">GraphX (Graph Processing)</a>
                             <a class="dropdown-item" href="{{ rel_path_to_root }}sparkr.html">SparkR (R on Spark)</a>
                             <a class="dropdown-item" href="{{ rel_path_to_root }}api/python/getting_started/index.html">PySpark (Python on Spark)</a>
+                            <a class="dropdown-item" href="{{ rel_path_to_root }}declarative-pipelines-programming-guide.html">Declarative Pipelines</a>
                         </div>
                     </li>