Add documentation for archive function. Closes #1396. (#1399)

docs/docs/content/archives.md

@@ -0,0 +1,32 @@
+# Archives
+
+A global public archive is maintained on the public web interface. It can be
+enabled under Settings -> Settings -> General -> Enable public mailing list
+archive.
+
+To make a campaign available in the public archive (provided it has been
+enabled in the settings as described above), enable the option
+'Publish to public archive' under Campaigns -> Create new -> Archive.
+
+When using template variables that depend on subscriber data (such as any
+template variable referencing `.Subscriber`), such data must be supplied
+as 'Campaign metadata', which is a JSON object that will be used in place
+of `.Subscriber` when rendering the archive template and content.
+
+When individual subscriber tracking is enabled, TrackLink requires that a UUID
+of an existing user is provided as part of the campaign metadata. Any clicks on
+a TrackLink from the archived campaign will be counted towards that subscriber.
+
+As an example:
+
+```json
+{
+  "UUID": "5a837423-a186-5623-9a87-82691cbe3631",
+  "email": "example@example.com",
+  "name": "Reader",
+  "attribs": {}
+}
+```
+
+![Archive campaign](images/archived-campaign-metadata.png)
+

docs/docs/mkdocs.yml

@@ -45,6 +45,7 @@ nav:
   - "Templating": templating.md
   - "Bounce processing": bounces.md
   - "Messengers": "messengers.md"
+  - "Archives": "archives.md"
   - "Internationalization": "i18n.md"
   - "Integrating with external systems": external-integration.md
   - "API": apis/apis.md