springboot Document how to initialize a database with R2DBC

Follow-up of #20715

Neither Flyway nor Liquibase have native support for R2DBC at the moment. One way to initialize a database using R2DBC is to create a DataSource for the duration of the migration.

Hopefully both of them have excellent support for that and we expose properties to let users configure the database to migrate. A caveat is that spring-jdbc is still required on the classpath so we should document all that and make sure it works as expected with smoke tests.