Fix docs target on Windows (git bash)

[btab]

The doc compilation dumps a huge amount of garbage to the terminal (complaining of an inability to create files with names that look like escape characters).

[btab]

Further interesting signal - we end up with the set of docs from the actual library missing (but all pony libs present).

The index.md file contains things like...

* [C:\Users\andre\src\pony-semver\semver](C:-Users-andre-src-pony-semver-semver--index.md)

What's the betting that those Windows path separators aren't being handled properly?

[SeanTAllen]

The docs target has the library when built on Linux. The docs target at this point exists really to create the docs for main.actor, so I'm not super concerned that it doesn't work for bash for windows as we aren't planning to use there and all our documentation generation from ponyc needs an overhaul.