Add platform-specific extension install paths

Enhanced extension installation documentation across docs, examples, and the generated install guide. Added Windows, macOS, and Linux-specific paths to help users find the correct installation directory. The dynamic README now shows the machine's actual extensions directory and typical OS paths. Also includes a minor fix removing wide string literal prefix from a path string.

examples/extensions/hello-nebula/README.txt

@@ -1,8 +1,13 @@
 Copy this folder into Nebula's extensions directory to install:
 
-  %LOCALAPPDATA%\Nebula\User Data\extensions\hello-nebula\
+  Windows:  %LOCALAPPDATA%\Nebula\User Data\extensions\hello-nebula\
+  macOS:    ~/Library/Application Support/Nebula/User Data/extensions/hello-nebula/
+  Linux:    ~/.local/share/Nebula/User Data/extensions/hello-nebula/
+            (or $XDG_DATA_HOME/Nebula/User Data/extensions/hello-nebula/)
 
-Then open Settings > Plugins and click Reload Plugins (or restart Nebula).
+Or open Settings > Plugins — the install path is shown there.
+
+Then click Reload Plugins (or restart Nebula).
 
 manifest.json fields:
   id, name, version, description, enabled

examples/extensions/return-youtube-dislike/README.txt

@@ -20,7 +20,12 @@ Install
 -------
 Copy this folder to:
 
-  %LOCALAPPDATA%\Nebula\User Data\extensions\return-youtube-dislike\
+  Windows:  %LOCALAPPDATA%\Nebula\User Data\extensions\return-youtube-dislike\
+  macOS:    ~/Library/Application Support/Nebula/User Data/extensions/return-youtube-dislike/
+  Linux:    ~/.local/share/Nebula/User Data/extensions/return-youtube-dislike/
+            (or $XDG_DATA_HOME/Nebula/User Data/extensions/return-youtube-dislike/)
+
+Or open Settings > Plugins — the install path is shown there.
 
 Open Settings > Plugins > Reload Plugins, then open a YouTube video.