Update internal mirror instructions (#14906)

Co-authored-by: Matt Mitchell <mmitche@microsoft.com>

Documentation/AzureDevOps/internal-mirror.md

@@ -1,25 +1,33 @@
-# dev.azure.com/dnceng Internal Mirror
+# Mirroring GitHub to dev.azure.com/dnceng and dev.azure.com/devdiv
 
-Public code should be mirrored to dev.azure.com/dnceng/internal (see [Azure DevOps Guidance](https://github.com/dotnet/arcade/blob/master/Documentation/AzureDevOps/AzureDevOpsGuidance.md)).  These are the steps for setting up your GitHub repo to mirror into dev.azure.com/dnceng/internal.
+Public code should be mirrored to dev.azure.com/dnceng/internal or dev.azure.com/dnceng/devdiv, depending on where your pipelines live. (see [Azure DevOps Guidance](https://github.com/dotnet/arcade/blob/master/Documentation/AzureDevOps/AzureDevOpsGuidance.md)).  These are the steps for setting up your GitHub repo for mirroring.
 
-1. Add the [maestro web hook](https://github.com/dotnet/arcade/blob/main/Documentation/Maestro/web-hooks.md)
-2. Make sure you have created a repo in the dev.azure.com/dnceng/internal project that is in the format "{org}-{repo}" (replace  any `/` with `-` in the GitHub repo name).
+1. Make sure you have a repo in the dev.azure.com/dnceng/internal project with a name in the format "{org}-{repo}" (replace  any `/` with `-` in the GitHub repo name). Please follow up with dnceng if a repository does not exist. For DevDiv repos, the pattern is {org}-{repo}-Trusted.
     - Example: github.com/dotnet/arcade => dotnet-arcade
-3. Create a PR to the dotnet/versions repo which adds data for repo and branches that you want mirrored to the [subscriptions json](https://github.com/dotnet/versions/blob/master/Maestro/subscriptions.json) file. Specifically, add a URI of the pattern `"https://github.com/{org}/{repo}/blob/{branch}/**/*"` for your GitHub repository to the `triggerPaths` list above `"action": "github-dnceng-azdo-mirror"`. Please alphabetize.
+2. Create a PR to the `dotnet-mirroring` internal repo which adds data for repo and branches that you want mirrored, to the [dnceng subscriptions json](https://dev.azure.com/dnceng/internal/_git/dotnet-mirroring?path=/dnceng-subscriptions.jsonc) or [devdiv subscriptions json](https://dev.azure.com/dnceng/internal/_git/dotnet-mirroring?path=/devdiv-subscriptions.jsonc) files. Specifically, add a URI for your GitHub repository to the `repos` object, then types of mirroring and regex branch patterns. Please alphabetize.
 
-```
-        "https://github.com/dotnet/project-system/blob/release/**/*",
-        "https://github.com/dotnet/toolset/blob/master/**/*",
-        "https://github.com/dotnet/toolset/blob/release/**/*",
-        "https://github.com/dotnet/roslyn/blob/master/**/*",
-        "https://github.com/dotnet/roslyn/blob/release/**/*",
-        "https://github.com/{org}/{repo}/blob/{branch}/**/*" // <-- insert your URI here, in alpha order.
-        "https://github.com/microsoft/msbuild/blob/master/**/*",
-        "https://github.com/microsoft/msbuild/blob/release/**/*",
+Example:
+```json
+    "https://github.com/dotnet/source-indexer": {
+      "fastForward": [
+        "main"
+      ]
+    },
+    "https://github.com/dotnet/sourcelink": {
+      "fastForward": [
+        "main", // Fast forward main -> main
+        "release/.*"
+      ]
+    },
+    "https://github.com/dotnet/spa-templates": {
+      "fastForward": [
+        // GitHubBranchNotFound "main",
+        "release/.*"
       ],
-      "action": "github-dnceng-azdo-mirror",
-      "actionArguments": {
-"vsoSourceBranch": "master",
+      "internalMerge": [
+        "release/.*" // Merge release/.* -> internal/release/.*
+      ]
+    },
 ```