Removed reference to deleted documentation and added language about updating to landing page (#697)

* Removed references to installing-from-release-windows.md removed in commit ea5aa6cee2

* Added language referencing the upgrade instructions in installation landing page

* Update doc/installation.md

---------

Co-authored-by: Markus Unterwaditzer <markus-github@unterwaditzer.net>

doc/SUMMARY.md

@@ -3,7 +3,6 @@
 [Introduction](./introduction.md)
 - [Installation](./installation.md)
   - [Installing from the latest release](./installing-from-release.md)
-    - [Installing from the latest release (Windows)](./installing-from-release-windows.md)
   - [Installing from source](./installing-from-source.md)
   - [Updating Rayhunter](./updating-rayhunter.md)
 - [Configuration](./configuration.md)

doc/installation.md

@@ -3,5 +3,8 @@
 So, you've got one of the [supported devices](./supported-devices.md), and are ready to start catching IMSI catchers. You have two options for installing Rayhunter:
 
 * [installing from a release (recommended)](./installing-from-release.md)
-  * [installing from a release on Windows](./installing-from-release-windows.md)
 * [installing from source](./installing-from-source.md)
+
+Already have Rayhunter installed but looking to update?
+
+* [Updating Rayhunter](./updating-rayhunter.md)