Install docs into /usr/share/doc/wcd-${PVR}.

diff -Nuar a/src/Makefile b/src/Makefile
--- a/src/Makefile	2019-08-14 10:07:15.000000000 +0000
+++ b/src/Makefile	2021-10-23 17:10:17.000000000 +0000
@@ -61,7 +61,7 @@
 datarootdir     = $(prefix)/share
 datadir         = $(datarootdir)
 
-docsubdir       = $(PACKAGE)-$(VERSION)
+docsubdir       = $(PACKAGE)-$(PVR)
 docdir          = $(datarootdir)/doc/$(docsubdir)
 localedir       = $(datarootdir)/locale
 sysconfdir      = /etc