Add docs

Author: davepagurek

docs/banner.md

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+# Banner
+
+A banner can optionally be added at the bottom of the page with a custom message and link.
+
+## Adding a new banner
+
+Edit the banner text in the body of `src/content/text-detail/en/banner.tsx`.
+
+Within the file's metadata:
+- Change the `title` metadata to a new slug for each message. This does not get displayed anywhere, but it gets recorded when a user dismisses the banner. This way, we can hide that banner for the user the next time they return, but still show them a new banner when we create a new message.
+- Make sure `hidden` is `true` for the banner to be visible
+
+## Removing a banner
+
+- Set `hidden` to `true` in `src/content/text-detail/en/banner.tsx`
+- Delete any translations of the banner that have been made in non-`en` locales.
+
+  This is to make sure that the next time a new banner is made, there is a clean slate to start from for translations.