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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README.md

@@ -30,7 +30,7 @@ Add the following to `/etc/hosts`:
 127.0.0.1 plan.thepeernet.com
 127.0.0.1 plan.turnsys.com
 127.0.0.1 plan.yourdreamnamehere.com
-127.0.0.1 startinglineproductions.com-bizopprodplan
+127.0.0.1 plan.startinglineproductions.com
 ```
 
 See `etc-hosts-entries.txt` for the complete list.
@@ -118,7 +118,7 @@ user/env/{fqdn}/
 | 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 (default) |
 
 ## Production Deployment