refactor: let updaters define the files they want to run on (#233)

This change reverses the responsibility for which files the updaters are
run on. Now each updater can specify the list of files and wether the
files should be created when they do not exist yet. This simplifies the
handling of each update in releaserpleaser.go, as we can just iterate
over all updaters and call it for each file of that updater.

Also update the flags to allow users to easily define which updaters
should run.

docs/reference/updaters.md

@@ -0,0 +1,33 @@
+# Updaters
+
+There are different updater for different purposes available.
+
+They each have a name and may be enabled by default. You can configure which updaters are used through the
+`updaters` input on GitHub Actions and GitLab CI/CD. This is a comma-delimited list of updaters that should be enabled, for updaters that are enabled by default you can remove them by adding a minus before its name:
+
+```
+updaters: -generic,packagejson
+```
+
+## Changelog
+
+- **Name**: `changelog`
+- **Default**: enabled
+
+This updater creates the `CHANGELOG.md` file and adds new release notes to it.
+
+## Generic Updater
+
+- **Name**: `generic`
+- **Default**: enabled
+
+This updater can update any file and only needs a marker on the line. It is enabled by default.
+
+Learn more about this updater in ["Updating arbitrary files"](../guides/updating-arbitrary-files.md).
+
+## Node.js `package.json` Updater
+
+- **Name**: `packagejson`
+- **Default**: disabled
+
+This updater can update the `version` field in Node.js `package.json` files. The updater is disabled by default.