[Docs] Uptime + process plugins

This patch adds a basic structure for plugin docs and provides
docs for the uptime and process plugins.

Change-Id: Id98e0ed8cd7ba29968ad16a0ce90ababe74498ea

doc/source/index.rst

@@ -7,8 +7,8 @@ that output data in various formats for different monitoring systems.
 Developers can quickly add new monitoring plugins (along with tests) without
 worrying about how to format the data.
 
-Documentation sections
+Overview
-----------------------
+--------
 
 .. toctree::
    :maxdepth: 2
@@ -17,3 +17,12 @@ Documentation sections
    configure
    usage
    develop
+
+Plugins
+-------
+
+.. toctree::
+   :maxdepth: 1
+   :glob:
+
+   plugin-*

doc/source/plugin-process.rst

@@ -0,0 +1,60 @@
+``process`` - monitor a running process
+=======================================
+
+The ``process`` plugin searches the list of running processes and checks
+whether a particular process is running.
+
+Usage
+-----
+
+The plugin takes only one argument: the name of the process to check. See the
+examples below for more details.
+
+Example
+-------
+
+In this example, the plugin searches the process list for the ``chronyd``
+process:
+
+.. code-block:: console
+
+    $ monitorstack process chronyd
+
+The default JSON output shows that the process was found:
+
+.. code-block:: json
+
+    {
+      "variables": {
+        "chronyd": 1
+      },
+      "message": "process check is ok",
+      "meta": {
+        "platform": "Linux"
+      },
+      "exit_code": 0,
+      "measurement_name": "process"
+    }
+
+Here's another example where the plugin searches for ``nginx``. That process is
+not running on this server:
+
+.. code-block:: console
+
+    $ monitorstack process nginx
+
+This time, the process is not found:
+
+.. code-block:: json
+
+    {
+      "variables": {
+        "nginx": 0
+      },
+      "message": "process failed -- Process nginx not found",
+      "meta": {
+        "platform": "Linux-4.10.8-200.fc25.x86_64-x86_64-with-fedora-25-Twenty_Five"
+      },
+      "exit_code": 1,
+      "measurement_name": "process"
+    }

doc/source/plugin-uptime.rst

@@ -0,0 +1,35 @@
+``uptime`` - get system uptime
+==============================
+
+The uptime plugin returns the number of seconds since the server was powered
+on.
+
+Usage
+-----
+
+This plugin takes no arguments.
+
+Example
+-------
+
+Run the plugin:
+
+.. code-block:: console
+
+    $ monitorstack uptime
+
+Example output in JSON format:
+
+.. code-block:: json
+
+    {
+      "variables": {
+        "uptime": "18570.18"
+      },
+      "message": "uptime is ok",
+      "meta": {
+        "platform": "Linux-4.10.8-200.fc25.x86_64-x86_64-with-fedora-25-Twenty_Five"
+      },
+      "exit_code": 0,
+      "measurement_name": "system_uptime"
+    }