stubs for initial DocC structure (#8572)

Placeholder structure for DocC content to lay out an initial area to
build into.

### Motivation:

@bripeticca and I wanted to have something in place to merge into and a
general plan of attack for shifting current content from the
`/documentation` directory into DocC format. This provides a bit more
structure over the bare docc catalog, and puts in placeholders for the
CLI documentation as well.

We'll be creating issues for each of the existing markdown files in
`/documentation`, and one for each cluster of CLI commands (grouped by
top-level subcommand) to track progress.

### Modifications:

This drops in place some light DocC content, with no real depth, to set
up an initial structure to merge content into. This isn't merging any
existing documentation

### Result:

The PackageManagerDocs target has more concrete content layout structure
to it, with placeholders for CLI commands and where to expand to build
out article sets based on existing documentation.

Use `swift package --disable-sandbox preview-documentation --target
PackageManagerDocs` with this PR to get a sense of the layout.

<img width="1416" alt="Screenshot 2025-04-29 at 2 19 10 PM"
src="https://github.com/user-attachments/assets/47875e36-c085-4bda-89a0-dcdde30f27c6"
/>

Author: heckj

Sources/PackageManagerDocs/Documentation.docc/Assets/command-icon.svg

@@ -0,0 +1,11 @@
+# Getting Started
+
+Learn to create and use a Swift package.
+
+## Overview
+
+overview content here....
+
+### First Section
+
+First section content

Sources/PackageManagerDocs/Documentation.docc/Assets/command-icon~dark.svg

@@ -0,0 +1,11 @@
+# Introducing Packages
+
+Learn to create and use a Swift package.
+
+## Overview
+
+overview content here....
+
+### First Section
+
+First section content

Sources/PackageManagerDocs/Documentation.docc/GettingStarted.md

@@ -0,0 +1,15 @@
+# swift package add-dependency
+
+Add a package dependency to the manifest.
+
+@Metadata {
+    @PageImage(purpose: icon, source: command-icon)
+}
+
+## Overview
+
+overview content here....
+
+### First Section
+
+First section content

Sources/PackageManagerDocs/Documentation.docc/IntroducingPackages.md

@@ -0,0 +1,15 @@
+# swift package add-product
+
+Add a new product to the manifest.
+
+@Metadata {
+    @PageImage(purpose: icon, source: command-icon)
+}
+
+## Overview
+
+overview content here....
+
+### First Section
+
+First section content

Sources/PackageManagerDocs/Documentation.docc/Package/PackageAddDependency.md

@@ -0,0 +1,15 @@
+# swift package add-target
+
+Add a new target to the manifest.
+
+@Metadata {
+    @PageImage(purpose: icon, source: command-icon)
+}
+
+## Overview
+
+overview content here....
+
+### First Section
+
+First section content

Sources/PackageManagerDocs/Documentation.docc/Package/PackageAddProduct.md

@@ -0,0 +1,15 @@
+# swift package add-target-dependency
+
+Add a new target dependency to the manifest.
+
+@Metadata {
+    @PageImage(purpose: icon, source: command-icon)
+}
+
+## Overview
+
+overview content here....
+
+### First Section
+
+First section content

Sources/PackageManagerDocs/Documentation.docc/Package/PackageAddTarget.md

@@ -0,0 +1,15 @@
+# swift package archive-source
+
+Create a source archive for the package.
+
+@Metadata {
+    @PageImage(purpose: icon, source: command-icon)
+}
+
+## Overview
+
+overview content here....
+
+### First Section
+
+First section content

Sources/PackageManagerDocs/Documentation.docc/Package/PackageAddTargetDependency.md

@@ -0,0 +1,15 @@
+# swift package clean
+
+Delete build artifacts.
+
+@Metadata {
+    @PageImage(purpose: icon, source: command-icon)
+}
+
+## Overview
+
+overview content here....
+
+### First Section
+
+First section content