packaging: install documentation to doc/ directory

The source directory is "doc" and "doc" is used by the WiX
installer. I think it makes sense to converge on the canonical
path name. While we're here, we also lower the case because
we can. (I think lower case names make more sense plus it is
more consistent.)

This only affects the Inno installer at the moment because
it is the only installer using the staging code.

Differential Revision: https://phab.mercurial-scm.org/D7162

contrib/packaging/hgpackaging/py2exe.py

@@ -41,8 +41,8 b' STAGING_RULES = ['
     ('dist/Microsoft.VC*.CRT.manifest', './'),
     ('dist/msvc*.dll', './'),
     ('dist/python*.dll', './'),
-    ('doc/*.html', 'Docs/'),
-    ('doc/style.css', 'Docs/'),
+    ('doc/*.html', 'doc/'),
+    ('doc/style.css', 'doc/'),
     ('mercurial/help/**/*.txt', 'help/'),
     ('mercurial/default.d/*.rc', 'default.d/'),
     ('mercurial/locale/**/*', 'locale/'),