Include install.txt and upgrading.txt in Sphinx documentation

doc/install.txt

@@ -55,8 +55,8 @@ First, install the required software.
    ClamAv from https://www.clamav.net/
 
 7. *Optional, for GNOME proxy setting parsing:*
-    PyGObject and GIO.
-    Best installed from your distribution e.g. ``python3-gi``
+   PyGObject and GIO.
+   Best installed from your distribution e.g. ``python3-gi``
 
 8. *Optional, to run the WSGI web interface:*
    Apache from https://httpd.apache.org/

doc/src/index.rst

@@ -20,6 +20,8 @@ Installation
 .. code-block:: console
 
    $ pip3 install git+https://github.com/linkchecker/linkchecker.git
+   
+See the :doc:`installation document <install>` for more information.
 
 Basic usage
 ------------
@@ -83,6 +85,8 @@ and test integration.
    :hidden:
 
    faq
+   install
+   upgrading
    man/linkchecker
    man/linkcheckerrc
    code/index

doc/src/install.rst

@@ -0,0 +1,7 @@
+Installation
+============
+If you are upgrading from older versions of LinkChecker you should
+also read the upgrading documentation stored in :doc:`upgrading`.
+
+.. include:: ../install.txt
+   :start-line: 5

doc/src/upgrading.rst

@@ -0,0 +1 @@
+.. include:: ../upgrading.txt

doc/upgrading.txt

@@ -1,7 +1,7 @@
 Upgrading
 =========
 Migrating from 9.x to 10.0
--------------------------
+--------------------------
 Python 3.5 or newer is required.
 
 The Python Beautiful Soup package is now required. A C compiler is not needed