Allow arbitrary data to be passed to templates

Documentation/templates.md

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+# Templates
+
+## Using your own templates
+
+Dex supports using your own templates and passing arbitrary data to them to help customize your installation.
+
+Steps:
+
+1. Copy contents of the `web` directory over to a new directory.
+2. Customize the templates as needed, be sure to retain all the existing variables so Dex continues working correctly.
+  a. Use this syntax `{{ "your_key" | extra }}` to use values from `frontend.extra`.
+3. Write a theme for your templates in the `themes` directory.
+4. Add your custom data to the Dex configuration `frontend.extra`.
+   ```yaml
+   frontend:
+     dir: /path/to/custom/web
+     extra:
+       tos_footer_link: "https://example.com/terms"
+       client_logo_url: "../theme/client-logo.png"
+       foo: "bar"
+   ```
+5. Set the `frontend.dir` value to your own `web` directory.
+
+To test your templates simply run Dex with a valid configuration and go through a login flow.