build: add doc files missing from dist tarball

The override.css/js files for sphinx docs were not being included into
the tarball created by `make dist`.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
(cherry picked from commit 26eaf6e)

Author: eqvinox

doc/developer/subdir.am

@@ -71,6 +71,7 @@ EXTRA_DIST += \
 	doc/developer/draft-zebra-00.ms \
 	doc/developer/ldpd-basic-test-setup.md \
 	doc/developer/release-announcement-template.md \
+	doc/developer/_static/overrides.css \
 	# end
 
 DEVBUILD = doc/developer/_build

doc/user/subdir.am

@@ -56,6 +56,8 @@ user_RSTFILES = \
 EXTRA_DIST += \
 	$(user_RSTFILES) \
 	doc/user/Useful_Sysctl_Settings.md \
+	doc/user/_static/overrides.css \
+	doc/user/_static/overrides.js \
 	# end
 
 USERBUILD = doc/user/_build