fixes for skip dir and adjustment to the docs

[tlienart]

@GiggleLiu would you mind trying this one? keeping things the same in your case it should be

servedocs(
    literate_dir=EXAMPLE_DIR,
    skip_dir=LITERATE_GENERATED_DIR
)

thanks for your report, I got confused at first but, indeed, the abspath was missing.

supersedes #168

[GiggleLiu]

Thank you for the fix! Will try it later.