docs: 📝 add Extensions to config guide

docs/guide/config.qmd

@@ -11,8 +11,8 @@ are available:
 -   `version`: The version of Data Package standard to check against.
     Defaults to `v2`.
 -   `exclusions`: A list of checks to exclude.
--   `custom_checks`: The list of custom checks to run in addition to the
-    checks defined in the standard.
+-   `extensions`: A list of extensions, which are additional checks
+    that supplement those specified by the Data Package standard.
 -   `strict`: Whether to include "SHOULD" checks in addition to "MUST"
     checks. Defaults to `False`.
 
@@ -97,13 +97,17 @@ the package and resource properties, and the resource `path` doesn't
 point to a data file. However, as we have defined exclusions for all of
 these, the function will flag no issues.
 
-## Adding custom checks
+## Adding extensions
 
-It is possible to create custom checks in addition to the ones defined
-in the Data Package standard.
+It is possible to add checks in addition to the ones defined in the
+Data Package standard. We call these additional checks *extensions*. There are currently two types of extensions
+supported: `CustomCheck` and `RequiredCheck`. You can add as many
+`CustomCheck`s and `RequiredCheck`s to your `Config` as you want to fit your needs.
+
+### Custom checks
 
 Let's say your organisation only accepts Data Packages licensed under
-MIT. You can express this requirement in a `CustomCheck` as follows:
+MIT. You can express this `CustomCheck` as follows:
 
 ```{python}
 license_check = cdp.CustomCheck(
@@ -117,24 +121,18 @@ license_check = cdp.CustomCheck(
 )
 ```
 
-Here's a breakdown of what each argument does:
-
--   `type`: An identifier for your custom check. This is what will show
-    up in error messages and what you will use if you want to exclude
-    your check. Each `CustomCheck` should have a unique `type`.
--   `jsonpath`: The location of the field or fields the custom check
-    applies to, expressed in [JSON
-    path](https://en.wikipedia.org/wiki/JSONPath) notation. This check
-    applies to the `name` field of all package licenses.
--   `message`: The message that is shown when the check is violated.
--   `check`: A function that expresses the custom check. It takes the
-    value at the `jsonpath` location as input and returns true if the
-    check is met, false if it isn't.
+For more details on what each parameter means, see the
+[`CustomCheck`](/docs/reference/custom_check.qmd) documentation.
+Specific to this example, the `type` is setting the identifier of the
+check to `only-mit` and the `jsonpath` is indicating to only check the
+`name` property of each license in the `licenses` property of the Data
+Package.
 
 To register your custom checks with the `check()` function, you add them
 to the `Config` object passed to the function:
 
 ```{python}
+#| eval: false
 package_properties = {
     "name": "woolly-dormice",
     "title": "Hibernation Physiology of the Woolly Dormouse: A Scoping Review.",
@@ -156,13 +154,39 @@ package_properties = {
     ],
 }
 
-config = cdp.Config(custom_checks=[license_check])
+config = cdp.Config(extensions=cdp.Extensions(custom_checks=[license_check]))
 cdp.check(properties=package_properties, config=config)
 ```
 
 We can see that the custom check was applied: `check()` returned one
 issue flagging the first license attached to the Data Package.
 
+### Required checks
+
+You can also set specific properties in the `datapackage.json` file to be
+required, even when they aren't required by the Data Package standard with a `RequiredCheck`.
+For example, if you want to make the `description` field of Data Package
+a required field, you can define a `RequiredCheck` like this:
+
+```{python}
+#| eval: false
+description_required = cdp.RequiredCheck(
+    jsonpath="$.description",
+    message="The 'description' field is required in the Data Package properties.",
+)
+```
+
+See the [`RequiredCheck`](/docs/reference/required_check.qmd)
+documentation for more details on its parameters.
+
+To apply this `RequiredCheck`, it should be added to the `Config` object passed to `check()` like shown below:
+
+```{python}
+#| eval: false
+config = cdp.Config(extensions=cdp.Extensions(required_checks=[description_required]))
+cdp.check(properties=package_properties, config=config)
+```
+
 ## Strict mode
 
 The Data Package standard includes properties that "MUST" and "SHOULD"