[backport] Add links to packaging and distro pages (#12603) [ci skip]

doc/lib.rst

@@ -208,6 +208,7 @@ Generic Operating System Services
   Its primary purpose is to produce output for Nimble packages,
   but it also contains the widely used **Distribution** enum
   that is useful for writing platform specific code.
+  See `packaging <packaging.html>`_ for hints on distributing Nim using OS packages.
 
 * `dynlib <dynlib.html>`_
   This module implements the ability to access symbols from shared libraries.

doc/packaging.rst

@@ -2,6 +2,9 @@
 Packaging Nim
 =============
 
+This page provide hints on distributing Nim using OS packages.
+
+See `distros <distros.html>`_ for tools to detect Linux distribution at runtime.
 
 Supported architectures
 -----------------------