Add javadoc -encoding UTF-8 to fix package.py unmappable char

[Quezion]

Fixes javadoc "unmappable character for encoding windows-1252", which was appearing during python scripts/package.py on my installation. Full error:

Generating JavaDoc target/delomboked to target/apidocs
target\delomboked\io\github\humbleui\jwm\EventMouseScroll.java:10: error: unmappable character (0x90) for encoding windows-1252
     * Direction Γå? + / ΓêÆ ΓåÆ
                   ^
target\delomboked\io\github\humbleui\jwm\EventMouseScroll.java:65: error: unmappable character (0x90) for encoding windows-1252
     * Direction Γå? + / ΓêÆ ΓåÆ
                   ^
target\delomboked\io\github\humbleui\jwm\ZOrder.java:9: error: unmappable character (0x9D) for encoding windows-1252
    /** Default ΓÇ£Always on topΓÇ? behavior */
                                  ^
3 errors
Traceback (most recent call last):
  File "C:\Users\Quest\repos\humbleui\JWM\script\package.py", line 38, in <module>
    main()
  File "C:\Users\Quest\repos\humbleui\JWM\script\package.py", line 32, in main
    build_utils.javadoc(["target/delomboked"], "target/apidocs", classpath=common.deps_compile())
  File "C:\Users\Quest\repos\humbleui\JWM\script\build_utils.py", line 145, in javadoc
    subprocess.check_call(["javadoc",
  File "C:\Python310\lib\subprocess.py", line 369, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['javadoc', '--class-path', 

Also tweaks windows/build.md to be slightly more specific. Should help guide unfamiliar users towards the correct way to compile under VS (#264)

[tonsky]

Thank you!