Migrates plugin documentation into DocC (#8766)

Migrate, and break up, content from `documentation/plugins.md` into DocC
articles.

resolves #8592

With the branch checked out, you can use the command:

```bash
swift package --disable-sandbox preview-documentation --target PackageManagerDocs
```

To build and display the collected documentation for easier reviewing.

---------

Co-authored-by: Bri Peticca <[email protected]>

Author: heckj

Sources/PackageManagerDocs/Documentation.docc/Documentation.md

@@ -30,10 +30,11 @@ The Swift Package Manager lets you share your code as a package, depend on and u
 - <doc:BundlingResources>
 - <doc:ReleasingPublishingAPackage>
 - <doc:ContinuousIntegration>
-- <doc:UsingShellCompletion>
-- <doc:SwiftPMAsALibrary>
+- <doc:Plugins>
 - <doc:ModuleAliasing>
 - <doc:PackageCollections>
+- <doc:UsingShellCompletion>
+- <doc:SwiftPMAsALibrary>
 
 <!-- ### Command Plugins -->
 <!-- placeholder for content about swift package manager extensions - command plugins -->

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

@@ -2,6 +2,7 @@
 
 @Metadata {
     @PageImage(purpose: icon, source: command-icon)
+    @Available("Swift", introduced: "5.6")
 }
 
 

Sources/PackageManagerDocs/Documentation.docc/PackageCollections.md

@@ -1,5 +1,9 @@
 # Package Collections
 
+@Metadata {
+    @Available("Swift", introduced: "5.5")
+}
+
 Learn to create, publish and use Swift package collections.
 
 ## Overview
@@ -332,7 +336,7 @@ A signed package collection has an extra `signature` object:
 ```
 
 - The signature string (represented by `"<SIGNATURE>"`) is used to verify the contents of the collection file haven't been tampered with since it was signed when SwiftPM user [adds the collection](<doc:PackageCollectionAdd#Signed-package-collections>) to their configured list of collections. It includes the certificate's public key and chain.
-- `certificate` contains details extracted from the signing certificate. `subject.commonName` should be consistent with the name of the publisher so that it's recognizable by users. The root of the certificate must be [installed and trusted on users' machines](<doc:PackageCollectionAdd#trusted-root-certificates>).
+- `certificate` contains details extracted from the signing certificate. `subject.commonName` should be consistent with the name of the publisher so that it's recognizable by users. The root of the certificate must be [installed and trusted on users' machines](<doc:PackageCollectionAdd#Trusted-root-certificates>).
 
 ### Requirements on signing certificate
 
@@ -349,7 +353,8 @@ Non-expired, non-revoked Swift Package Collection certificates from [developer.a
 
 With the `package-collection-sign` tool, the root certificate provided as input for signing a collection is automatically trusted. When SwiftPM user tries to add the collection, however,
 the root certificate must either be preinstalled with the OS (Apple platforms only) or found in the `~/.swiftpm/config/trust-root-certs` directory (all platforms) or shipped with 
-the [certificate-pinning configuration](<doc:#Protecting-package-collections>), otherwise the [signature check](<doc:PackageCollectionAdd#Signed-package-collections>) will fail. Collection publishers should make the DER-encoded 
+the [certificate-pinning configuration](<doc:#Protecting-package-collections>), otherwise the [signature check](<doc:PackageCollectionAdd#Signed-package-collections>) will fail.
+Collection publishers should make the DER-encoded 
 root certificate(s) that they use downloadable so that users can adjust their setup if needed.
 
 

Sources/PackageManagerDocs/Documentation.docc/PackageCollections/PackageCollectionAdd.md

@@ -2,6 +2,7 @@
 
 @Metadata {
     @PageImage(purpose: icon, source: command-icon)
+    @Available("Swift", introduced: "5.5")
 }
 
 Add a new collection.

Sources/PackageManagerDocs/Documentation.docc/PackageCollections/PackageCollectionDescribe.md

@@ -2,6 +2,7 @@
 
 @Metadata {
     @PageImage(purpose: icon, source: command-icon)
+    @Available("Swift", introduced: "5.5")
 }
 
 Get metadata for a collection or a package included in an imported collection.

Sources/PackageManagerDocs/Documentation.docc/PackageCollections/PackageCollectionList.md

@@ -2,6 +2,7 @@
 
 @Metadata {
     @PageImage(purpose: icon, source: command-icon)
+    @Available("Swift", introduced: "5.5")
 }
 
 List configured collections.

Sources/PackageManagerDocs/Documentation.docc/PackageCollections/PackageCollectionRefresh.md

@@ -2,6 +2,7 @@
 
 @Metadata {
     @PageImage(purpose: icon, source: command-icon)
+    @Available("Swift", introduced: "5.5")
 }
 
 Refresh configured collections.

Sources/PackageManagerDocs/Documentation.docc/PackageCollections/PackageCollectionRemove.md

@@ -2,6 +2,7 @@
 
 @Metadata {
     @PageImage(purpose: icon, source: command-icon)
+    @Available("Swift", introduced: "5.5")
 }
 
 Remove a configured collection.

Sources/PackageManagerDocs/Documentation.docc/PackageCollections/PackageCollectionSearch.md

@@ -2,6 +2,7 @@
 
 @Metadata {
     @PageImage(purpose: icon, source: command-icon)
+    @Available("Swift", introduced: "5.5")
 }
 
 Search for packages by keywords or module names.

Sources/PackageManagerDocs/Documentation.docc/PackageRegistry/PackageRegistryLogin.md

@@ -2,6 +2,7 @@
 
 @Metadata {
     @PageImage(purpose: icon, source: command-icon)
+    @Available("Swift", introduced: "5.9")
 }
 
 Log in to a registry.