Clean up lua docs scripts

- ldoc is now a git submodule
- ldoc can be installed using scripts/install.{sh,ps1}
- lua docs can be build using scripts/build.{sh,ps1}
- all lua docs-related files now reside inside of the docs folder

.gitignore

@@ -51,7 +51,6 @@ desktop.ini
 # Merge script
 temp.txt
 
-docs/html
 # Private assets
 Barotrauma/BarotraumaShared/Content/*
 Barotrauma/**/GameAnalyticsKeys.cs