replace illegal chars in entity path (Windows)

[michelou]

For instance the entity path <empty> is not legal on Windows:

testing ./bin/dotd
Compiling (1/1): HelloWorld.scala
[doc info] Generating doc page for: <empty>
Exception in thread "main" java.nio.file.InvalidPathException: Illegal char <<> at index 59: C:\Users\appveyor\AppData\Local\Temp\1\dotty_out\_site/api/<empty>/index.html
	at sun.nio.fs.WindowsPathParser.normalize(WindowsPathParser.java:182)
	at sun.nio.fs.WindowsPathParser.parse(WindowsPathParser.java:153)
	at sun.nio.fs.WindowsPathParser.parse(WindowsPathParser.java:77)
	at sun.nio.fs.WindowsPath.parse(WindowsPath.java:94)
	at sun.nio.fs.WindowsFileSystem.getPath(WindowsFileSystem.java:255)
	[...]
	at scala.compat.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dottydoc.staticsite.Site.createOutput(Site.scala:186)
	at dotty.tools.dottydoc.staticsite.Site.generateApiDocs(Site.scala:230)
	at dotty.tools.dottydoc.DocDriver.process(DocDriver.scala:52)
	at dotty.tools.dotc.Driver.process(Driver.scala:135)
	at dotty.tools.dotc.Driver.process(Driver.scala:147)
	at dotty.tools.dotc.Driver.main(Driver.scala:174)
	at dotty.tools.dottydoc.Main.main(Main.scala)

[dottybot]

Hello, and thank you for opening this PR! 🎉

All contributors have signed the CLA, thank you! ❤️

Have an awesome day! ☀️

[liufengyun]

LGTM, thanks a lot for your contribution @michelou

[liufengyun]

Do we need to change "/" as well?

[michelou]

We need to check if mkdirs() (and other functions used in doc-tool) do support both kinds of file separator. If yes, we can save time and extra work (besides keeping the code shorter) !

[liufengyun]

Yes, if necessary we can do in another PR.