docs: Add cloud-init overview/introduction (#4440)

One thing cloud-init has been lacking is a high-level overview of
what it is, what it does, and how it works. Once we are happy with
the general structure and content of this introduction, I will also
add links to the various pages around the docs that a newcomer might
want to visit for more information on each of the topics.

doc/rtd/index.rst

@@ -81,6 +81,7 @@ projects, contributions, suggestions, fixes and constructive feedback.
    :hidden:
    :maxdepth: 2
 
+   Introduction <introduction.rst>
    tutorial/index
    howto/index
    explanation/index

doc/rtd/introduction.rst

@@ -0,0 +1,116 @@
+Introduction to cloud-init
+**************************
+
+Managing and configuring cloud instances and servers can be a complex
+and time-consuming task. Cloud-init is an open source initialisation tool that
+was designed to make it easier to get your systems up and running with a
+minimum of effort, already configured according to your needs.
+
+It’s most often used by developers, system administrators and other IT
+professionals to automate configuration of VMs, cloud instances, or machines on
+a network. For brevity, we’ll refer only to instances for the rest of this
+page, but assume we’re also including bare metal and VMs in the discussion as
+well. By automating routine setup tasks, cloud-init ensures repeatability and
+efficiency in your system provisioning.
+
+What is the benefit of cloud-init?
+==================================
+
+When you deploy a new cloud instance, cloud-init takes an initial configuration
+that you supply, and it automatically applies those settings when the instance
+is created. It’s rather like writing a to-do list, and then letting cloud-init
+deal with that list for you.
+
+The real power of cloud-init comes from the fact that you can re-use your
+configuration instructions as often as you want, and always get consistent,
+reliable results. If you’re a system administrator and you want to deploy a
+whole fleet of machines, you can do so with a fraction of the time and effort
+it would take to manually provision them.
+
+What does cloud-init do?
+========================
+
+Cloud-init can handle a range of tasks that normally happen when a new instance
+is created. It's responsible for activities like setting the hostname,
+configuring network interfaces, creating user accounts, and even running
+scripts. This streamlines the deployment process; your cloud instances will all
+be automatically configured in the same way, which reduces the chance to
+introduce human error.
+
+How does cloud-init work?
+=========================
+
+The operation of cloud-init broadly takes place in two separate phases during
+the boot process. The first phase is during the early (local) boot stage,
+before networking has been enabled. The second is during the late boot stages,
+after cloud-init has applied the networking configuration.
+
+During early boot
+-----------------
+
+In this pre-networking stage, cloud-init discovers the datasource, obtains
+all the configuration data from it, and configures networking. In this phase,
+it will:
+
+* **Identify the datasource**:
+  The hardware is checked for built-in values that will identify the datasource
+  your instance is running on. The datasource is the source of all
+  configuration data.
+* **Fetch the configuration**:
+  Once the datasource is identified, cloud-init fetches the configuration data
+  from it. This data tells cloud-init what actions to take. This can be in the
+  form of:
+
+  * **Metadata** about the instance, such as the machine ID, hostname and
+    network config, or
+  * **Vendor data** and/or **user data**. These take the same form, although
+    Vendor data is provided by the cloud vendor, and user data is provided by
+    the user. These data are usually applied in the post-networking phase, and
+    might include:
+
+    * Hardware optimisations
+    * Integration with the specific cloud platform
+    * SSH keys
+    * Custom scripts
+
+* **Write network configuration**:
+  Cloud-init writes the network configuration and configures DNS, ready to be
+  applied by the networking services when they come up.
+
+During late boot
+----------------
+
+In the boot stages that come after the network has been configured, cloud-init
+runs through the tasks that were not critical for provisioning. This is where
+it configures the running instance according to your needs, as specified in the
+vendor data and/or user data. It will take care of:
+
+* **Configuration management**:
+  Cloud-init can interact with tools like Puppet, Ansible, or Chef to apply
+  more complex configuration - and ensure the system is up-to-date.
+* **Installing software**:
+  Cloud-init can install software at this stage, and run software updates to
+  make sure the system is fully up-to-date and ready to use.
+* **User accounts**:
+  Cloud-init is able to create and modify user accounts, set default passwords,
+  and configure permissions.
+* **Execute user scripts**:
+  If any custom scripts were provided in the user data, cloud-init can run
+  them. This allows additional specified software to be installed, security
+  settings to be applied, etc. It can also inject SSH keys into the instance’s
+  ``authorized_keys`` file, which allows secure remote access to the machine.
+
+After this stage is complete, your instance is fully configured!
+
+What's next?
+============
+
+Now that you have an overview of the basics of what cloud-init is, what it does
+and how it works, you will probably want to
+:ref:`try it out for yourself<tutorial_qemu>`.
+
+You can also read in more detail about what cloud-init does
+:ref:`during the different boot stages<boot_stages>`, and the
+:ref:`types of configuration<configuration>` you can pass to cloud-init and
+how they're used.
+