docs: explain how to use presets

docs/usage/configuration-options.md

@@ -855,6 +855,7 @@ The above would mean Renovate would not include files matching the above glob pa
 ## extends
 
 See [shareable config presets](https://docs.renovatebot.com/config-presets) for details.
+Learn how to use presets by reading the [Key concepts, Presets](../usage/key-concepts/presets.md/#how-to-use-presets) page.
 
 ## extractVersion
 

docs/usage/key-concepts/presets.md

@@ -16,6 +16,20 @@ Use presets to:
 - Share your configuration with others
 - Use somebody else's configuration and extend it with your own rules
 
+## How to use presets
+
+Let's say you're using the `config:base` preset, and want to pin your GitHub Action digests.
+Instead of writing your own Renovate config, you search through Renovate's built-in presets.
+You find the the `helpers:pinGitHubActionDigests` preset and add it to the `extends` array:
+
+```json
+{
+  "extends": ["config:base", "helpers:pinGitHubActionDigests"]
+}
+```
+
+Renovate now follows the rules for `config:base` plus the rules for `helpers:pinGitHubActionDigests`.
+
 ## Managing config for many repositories
 
 If you manage Renovate for many repositories, then you should create a global preset configuration.
@@ -32,6 +46,12 @@ Renovate comes with a lot of built-in presets that you can use.
 Browse [Renovate's default presets](https://docs.renovatebot.com/presets-default/) to find any that are useful to you.
 Once you find a preset you like, put it in an `extends` array in your config file.
 
+### Contributing a new built-in preset
+
+If you have a Renovate config that may help others, you can put it into Renovate's built-in presets.
+
+Read [Contributing to presets](https://docs.renovatebot.com/config-presets/#contributing-to-presets) to learn how.
+
 ## Summary
 
 In short: