Configuration validation

Validate the Kamal configuration giving useful warning on errors.
Each section of the configuration has its own config class and a YAML
file containing documented example configuration.

You can run `kamal docs` to see the example configuration, and
`kamal docs <section>` to see the example configuration for a specific
section.

The validation matches the configuration to the example configuration
checking that there are no unknown keys and that the values are of
matching types.

Where there is more complex validation - e.g for envs and servers, we
have custom validators that implement those rules.

Additonally the configuration examples are used to generate the
configuration documentation in the kamal-site repo.

You generate them by running:

```
bundle exec bin/docs <kamal-site-checkout>
```

lib/kamal/configuration/docs/boot.yml

@@ -0,0 +1,19 @@
+# Booting
+#
+# When deploying to large numbers of hosts, you might prefer not to restart your services on every host at the same time.
+#
+# Kamal’s default is to boot new containers on all hosts in parallel. But you can control this with the boot configuration.
+
+# Fixed group sizes
+#
+# Here we boot 2 hosts at a time with a 10 second gap between each group.
+boot:
+  limit: 2
+  wait: 10
+
+# Percentage of hosts
+#
+# Here we boot 25% of the hosts at a time with a 2 second gap between each group.
+boot:
+  limit: 25%
+  wait: 2