Adding Codespace prebuilds to docs-internal (#23952)

* Adding Codespaces prebuilds
* Adding permissions
* Use pinned SHA for github/codespaces-precache action
* Add comments
* Add codespaces-precache repo to reference allow list

Co-authored-by: James M. Greene <jamesmgreene@github.com>

.github/workflows/codespaces-prebuild.yml

@@ -0,0 +1,30 @@
+name: Prebuild Codespaces
+
+# **What it does**: Prebuild the Codespaces image using powerful machines.
+#   See https://github.com/github/codespaces-precache#readme for more details.
+#   IMPORTANT: Requires we set a `EXPERIMENTAL_CODESPACE_CACHE_TOKEN` Codespaces
+#   Secret (NOT an Actions Secret) in the repository.
+# **Why we have it**: Reduces startup time when booting Codespaces.
+# **Who does it impact**: Any Docs contributors who want to use Codespaces.
+
+on:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
+
+# Currently requires write, but in the future will only require read
+permissions:
+  contents: write
+
+jobs:
+  createPrebuild:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@ec3a7ce113134d7a93b817d10a8272cb61118579
+      - uses: github/codespaces-precache@2ad40630d7e3e45e8725d6a74656cb6dd17363dc
+        with:
+          regions: WestUs2 EastUs WestEurope SouthEastAsia
+          sku_name: basicLinux32gb
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}