feat(plugin-search): Add a button which allows to index all existing documents

[devj3ns]

When installing the Search Plugin (https://payloadcms.com/docs/plugins/search) on an existing Payload instance with several collections and documents, I’ve noticed that the search collection initially remains empty. Entries are only added to the search collection when a document is manually updated or newly added.

What is missing from the plugin is a way to automatically index all existing documents (for each locale) into the search collection. This could be implemented as a button on the search collection.

See also https://discord.com/channels/967097582721572934/1004009734115966976/1300523364133371934