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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scripts/configure-sites.sh

@@ -22,7 +22,7 @@ declare -A SITES=(
     ["plan.thepeernet.com"]="The Peer Network Business Plan"
     ["plan.turnsys.com"]="TSYS Business Plan"
     ["plan.yourdreamnamehere.com"]="Your Dream Name Here Business Plan"
-    ["startinglineproductions.com-bizopprodplan"]="Starting Line Productions Business Plan"
+    ["plan.startinglineproductions.com"]="Starting Line Productions Business Plan"
     ["staticsites.turnsys.com"]="TSYS Static Sites"
 )