feat(dashboard): on demand config migration (#31129)

Co-authored-by: HonkingGoose <34918129+HonkingGoose@users.noreply.github.com>
Co-authored-by: Rhys Arkins <rhys@arkins.net>
Co-authored-by: Sergei Zharinov <zharinov@users.noreply.github.com>

docs/usage/config-migration.md

@@ -0,0 +1,80 @@
+# Config Migration
+
+As part of continuous improvement and refinement, the Renovate maintainers often rename, remove or combine configuration options.
+
+When the Renovate maintainers change configuration options, they add "config migration" code.
+The migration code allows "legacy" config from users to keep working.
+Config migration works by migrating legacy config internally, before the config is used.
+If done right, config migration "just works" silently and legacy configs continue working indefinitely.
+The only sign that "config migration" is needed is the debug message in the Renovate logs, noting the old and newly migrated configs.
+
+## Enabling config migration pull requests
+
+Even though Renovate allows you to keep using "legacy config", we recommend you update the configuration names in your config regularly.
+Using the latest names:
+
+- makes it easier for you to understand the config
+- helps you find the documentation for the config
+
+Renovate can create a config migration Pull Request, to migrate legacy config in your configuration file.
+To get automated config migration Pull Requests from Renovate: set the [`configMigration`](./configuration-options.md#configmigration) config option to `true`.
+
+Config migration PRs are disabled by default.
+But we recommend you enable config migration PRs, because:
+
+- the config migration PR "tells" you something changed
+- up-to-date terms help you search the Renovate documentation
+- up-to-date terms help you, and us, debug problems quicker
+
+## Config migration scenarios
+
+The scenarios for config migration are:
+
+- No config migration needed
+- Config migration needed, and enabled
+- Config migration needed, but disabled
+- Config migration needed, but there is a previously closed migration PR
+
+### No config migration needed
+
+Renovate takes no action.
+
+### Config migration needed, and enabled
+
+Renovate will:
+
+1. Create a Config Migration PR
+1. If the Dependency Dashboard issue is enabled, then Renovate puts a link to the Config Migration PR on the dashboard
+
+### Config migration needed, but disabled
+
+If config migration is needed, but disabled then Renovate adds a checkbox to the Dependency Dashboard (if the dashboard exists).
+This is known as "on-demand" config migration, because migration PRs are only created at the request of the user by ticking the checkbox.
+
+The checkbox looks like this:
+
+```
+- [ ] Select this checkbox to let Renovate create an automated Config Migration PR.
+```
+
+When you select the checkbox:
+
+1. Renovate creates a config migration PR
+2. Renovate replaces the checkbox with a link to the Config Migration PR
+
+For example:
+
+```
+See Config Migration PR: #1.
+```
+
+### Config migration needed, but there is a closed migration PR
+
+In this case, it does not matter if Config Migration is enabled, or not.
+Renovate will:
+
+- Add a checkbox to the Dependency Dashboard issue (if enabled)
+- When you select the checkbox on the dashboard, Renovate will:
+  1. Delete the _old_ config migration branch
+  1. Create a _new_ config migration PR
+  1. Replace the checkbox with a link to the _new_ PR in the Dependency Dashboard issue

docs/usage/configuration-options.md

@@ -582,11 +582,7 @@ After we changed the [`baseBranches`](#basebranches) feature, the Renovate confi
     When downgrading JSON5 to JSON Renovate may also remove the JSON5 comments.
     This can happen because Renovate wrongly converts JSON5 to JSON, thus removing the comments.
 
-<!-- prettier-ignore -->
-!!! note
-    When you close a config migration PR, Renovate ignores it forever.
-    This also means that Renovate won't create a config migration PR in future.
-    If you closed the PR by accident, find the closed PR and re-name the PR title to get a new PR.
+For more details, read the [config migration documentation](./config-migration.md).
 
 ## configWarningReuseIssue