Fix bug with linking to Muse project files when muse-file-extension is nil

Fix bug with linking to Muse project files when muse-file-extension is nil

* lisp/muse-project.el (muse-project-resolve-directory): New function
  that figures out the directory part of the path that provides a link to
  a page.
  (muse-project-resolve-link): Expand docstring.  Use the new function.
  Don't call muse-publish-link-file, because that does the wrong thing
  when we have muse-file-extension set to nil.  Thanks to Sebastian
  Obermanns for the report.

* lisp/muse-publish.el (muse-publish-link-name, muse-publish-link-file)
  (muse-publish-link-page): Add docstrings so that I can remember what
  each of these things do.

* lisp/muse.el (muse-page-name): Improve docstring.