Add information about /liquibase and /flyway to Actuator's docs

Closes gh-3438

spring-boot-actuator-docs/src/main/asciidoc/flyway.adoc

@@ -0,0 +1,12 @@
+=== /flyway
+This endpoint provides information about database migrations that have been performed
+by Flyway.
+
+Example curl request:
+include::{generated}/flyway/curl-request.adoc[]
+
+Example HTTP request:
+include::{generated}/flyway/http-request.adoc[]
+
+Example HTTP response:
+include::{generated}/flyway/http-response.adoc[]

spring-boot-actuator-docs/src/main/asciidoc/liquibase.adoc

@@ -0,0 +1,12 @@
+=== /liquibase
+This endpoint provides information about database migrations that have been performed
+by Liquibase.
+
+Example curl request:
+include::{generated}/liquibase/curl-request.adoc[]
+
+Example HTTP request:
+include::{generated}/liquibase/http-request.adoc[]
+
+Example HTTP response:
+include::{generated}/liquibase/http-response.adoc[]

spring-boot-actuator-docs/src/restdoc/resources/db/changelog/db.changelog-master.yaml

@@ -0,0 +1,5 @@
+databaseChangeLog:
+  - changeSet:
+      id: 1
+      author: awilkinson
+      changes: