Add docs for security models (#9)

- Change default security to CONFIGURE_ME
- Fix log on return URL handling
- Update INSTALLING security model descriptions

src/start.php

@@ -60,10 +60,10 @@ function page_head(string $title): void {
     <div><a class=title href="/">Feed Reader Central</a><span class=version>v<?=$version?></span></div>
     <div><?php
         if (array_key_exists(Key::USER_ID, $_SESSION)) {
-            echo '<a href=/feed?id=new>Add Feed</a> | <a href=/user/log-off>Log Off</a>';
+            echo '<a href=/feed?id=new>Add Feed</a> | <a href=/docs/>Docs</a> | <a href=/user/log-off>Log Off</a>';
             if ($_SESSION[Key::USER_EMAIL] != Security::SINGLE_USER_EMAIL) echo " | {$_SESSION[Key::USER_EMAIL]}";
         } else {
-            echo '<a href=/user/log-on>Log On</a>';
+            echo '<a href=/user/log-on>Log On</a> | <a href=/docs/>Docs</a>';
         } ?>
     </div>
 </header>