Add installation page to docs (#856)

Add a dedicated installation guide with a single pip command and
link it from the handbook index and quick start introduction.

Author: PeterDaveHello

docs/source/index.rst

@@ -31,6 +31,7 @@ The Handbook
 .. toctree::
    :maxdepth: 2
 
+   installation
    introduction
    jobs
    filters

docs/source/installation.rst

@@ -0,0 +1,30 @@
+.. _installation:
+
+Installation
+============
+
+For prerequisites and optional dependencies, see :doc:`dependencies`.
+
+Install or upgrade with pip
+---------------------------
+
+Use this command for both first-time installs and upgrades:
+
+.. code-block:: bash
+
+    python -m pip install --upgrade urlwatch
+
+Verify the installation
+-----------------------
+
+Check that urlwatch is on your ``PATH`` and prints a version number:
+
+.. code-block:: bash
+
+    urlwatch --version
+
+Next steps
+----------
+
+Continue with :doc:`introduction` for the Quick Start and initial
+configuration.

docs/source/introduction.rst

@@ -7,6 +7,8 @@ Introduction
 Quick Start
 -----------
 
+First, install urlwatch by following :doc:`installation`. Once installed:
+
 1. Run ``urlwatch`` once to migrate your old data or start fresh
 2. Use ``urlwatch --edit`` to customize jobs and filters (``urls.yaml``)
 3. Use ``urlwatch --edit-config`` to customize settings and reporters (``urlwatch.yaml``)