Clean up installed docs

All old, outdated, manually-generated translations of docs now
go into an "outdated" subdirectory.  The "tuxpaint-docs"/gettext-based
ones are the "first-class citizens".

Don't install the docs directory's "Makefile", or the instructions
for how to release Tux Paint, as those are not necessary to end users.

docs/README.txt

@@ -1,2 +1,6 @@
-See [locale]/html/README.html or [locale]/README.txt.
+See [locale]/html/README.html or [locale]/README.txt,
+and the other documentation within.
+
+Older, outdated translations of documentation reside in
+the "outdated/" subdirectory.