help-docs: Move help center documentation to top level directory.

These files are not Jinja2 templates, so there's no reason that they needed
to be inside `templates/zerver`. Moving them to the top level reflects their
importance and also makes it feel nicer to work on editing the help center content, 
without it being unnecessary buried deep in the codebase.

help/enable-full-width-display.md

@@ -0,0 +1,18 @@
+# Enable full width display
+
+By default, Zulip limits the width of the center message pane, because it's
+easier and faster to read paragraphs that don't have long lines. This is
+also why many news sites, blogs, and social media platforms limit their
+paragraph width.
+
+You can instead configure Zulip to use the full width of wide screens.
+
+### Enable full width display
+
+{start_tabs}
+
+{settings_tab|display-settings}
+
+1. Under **Advanced**, select **Use full width on wide screens**.
+
+{end_tabs}