treewide: move everything theme related to submodule

All themes are moved to `_theme` submodule, which will be shared with
other websites on hacks.guide.

Repository-specific files can be overlayed after the fact, such as
additional languages, custom JavaScript, progress table paths and the
like.

.github/workflows/publish.yml

@@ -13,6 +13,7 @@ jobs:
       - uses: actions/checkout@v3
         with:
           persist-credentials: false
+          submodules: recursive
 
       - uses: ruby/setup-ruby@v1
         with:

.github/workflows/test.yml

@@ -12,6 +12,7 @@ jobs:
       - uses: actions/checkout@v3
         with:
           persist-credentials: false
+          submodules: recursive
 
       - uses: ruby/setup-ruby@v1
         with: