docs: add missing javacpp native jar to install guide (v0.4.3)

JavaCPP Loader (javacpp.jar) ships pure-Java code that needs its own
JNI bridge DLL (jnijavacpp) to extract & link other native libraries.
That bridge lives in the platform-specific javacpp-<os>-<arch>.jar
which the old install guide silently omitted — users following it
ended up with a black panel and an UnsatisfiedLinkError for jnijavacpp
because FFmpeg natives could never be loaded.

This bumps the required JavaCV jar list from 4 to 5, updates the
-Xbootclasspath/a: examples on all three OSes, and adds a diagnostic
note for the matching log line.

Docs-only change; mod code is unchanged from v0.4.2.

gradle.properties

@@ -5,7 +5,7 @@ org.gradle.configuration-cache=false
 
 # Mod
 mod_id=video_player
-mod_version=0.4.2
+mod_version=0.4.3
 maven_group=com.ejclaw.videoplayer
 archives_base_name=video_player