fix: make sphinxdocs support directory inputs

[rickeylev]

The logic to relocate files assumed that all the inputs were plain file artifacts. When
a directory artifact was used, then ctx.actions.symlink() would fail because it requires
the output artifact and input target artifact to be the same type of file (plain file or
directory).

To fix, use File.is_directory to detect if the input is a directory or file, then call
declare_file() or declare_directory() as appropriate. The later symlink() call is
then happy the two args match.

Fixes #2374