[SPARK-51991] Add SparkConnect.md, GettingStarted.md and SparkSession.md

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

This PR aims to add a library document.
- `SparkConnect.md`
- `GettingStarted.md`
- `SparkSession.md`

### Why are the changes needed?

This document will be served in [Swift Package Index](https://swiftpackageindex.com/apache/spark-connect-swift) repository like the following.

<img width="766" alt="Screenshot 2025-05-03 at 9 10 51 AM" src="https://github.com/user-attachments/assets/fd409f01-a3a3-41f8-99cb-f57a99e29204" />

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

No.

### How was this patch tested?

Manual review.

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

No.

Closes #107 from dongjoon-hyun/SPARK-51991.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

.spi.yml

@@ -15,3 +15,6 @@
 version: 1
 metadata:
   authors: "Apache Spark project"
+builder:
+  configs:
+    - documentation_targets: [SparkConnect]

Package.swift

@@ -47,6 +47,9 @@ let package = Package(
         .product(name: "GRPCProtobuf", package: "grpc-swift-protobuf"),
         .product(name: "GRPCNIOTransportHTTP2", package: "grpc-swift-nio-transport"),
         .product(name: "FlatBuffers", package: "flatbuffers"),
+      ],
+      resources: [
+        .process("Documentation.docc")
       ]
     ),
     .executableTarget(