refactor: rename startinglineproductions to plan.startinglineproductions.com

- Rename mdbook source directory
- Rename Grav environment directory in container
- Update setup.php hostname mapping
- Update configure-sites.sh
- Update etc-hosts-entries.txt
- Update README.md site inventory

Consistent naming convention: all plan sites now use plan.{domain}

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setup.php

@@ -34,7 +34,7 @@ $hostMap = [
     'plan.thepeernet.com' => 'plan.thepeernet.com',
     'plan.turnsys.com' => 'plan.turnsys.com',
     'plan.yourdreamnamehere.com' => 'plan.yourdreamnamehere.com',
-    'startinglineproductions.com-bizopprodplan' => 'startinglineproductions.com-bizopprodplan',
+    'plan.startinglineproductions.com' => 'plan.startinglineproductions.com',
     'staticsites.turnsys.com' => 'staticsites.turnsys.com',
 ];