Remove windows installer directions, update disrributions

We no longer supply a windows installer. Removed section describing
it.

Replace debian with Gentoo distribution and verified that NetBSD still
offers roundup (1.4.19) with it's packging tools. Also added note that
distributions may have old versions and you should get it from the
roundup web site or pypi.

Author: rouilj

doc/installation.txt

@@ -49,15 +49,6 @@ Some variants of Linux will need an additional "python dev" package
 installed for Roundup installation to work. Debian and derivatives, are
 known to require this.
 
-If you are using the Roundup Windows installer on a 64 bit system and
-you get the error message::
-
-  "No Python installation found in the registry"
-
-you need to install a 32 bit version of python. The 64 bit versions
-use a different registry key that the installer doesn't detect. See:
-https://issues.roundup-tracker.org/issue2550718 for details.
-
 Optional Components
 ===================
 
@@ -154,10 +145,12 @@ Getting Roundup
 ===============
 
 .. note::
-    Some systems, such as Debian and NetBSD, already have Roundup
+    Some systems, such as Gentoo and NetBSD, already have Roundup
     installed. Try running the command "roundup-admin" with no arguments,
     and if it runs you may skip the `Basic Installation Steps`_
     below and go straight to `configuring your first tracker`_.
+    However they may install an old version, so you are probably
+    beter off installing it from the roundup web site or pypi.
 
 Download the latest version from https://www.roundup-tracker.org/.