docs: Consolidate all R2DBC docs onto one page. (#1606)

Consolidate all R2DBC docs into a single page for easier maintenance. Part of #1550

Author: hessjcg

.github/release-please.yml

@@ -20,6 +20,4 @@ extraFiles:
  - docs/jdbc-mysql.md
  - docs/jdbc-postgres.md
  - docs/jdbc-sqlserver.md
- - docs/r2dbc-mysql.md
- - docs/r2dbc-postgres.md
- - docs/r2dbc-sqlserver.md
+ - docs/r2dbc.md

README.md

@@ -16,9 +16,7 @@ JDBC:
 * [Connecting to SQL Server using JDBC](docs/jdbc-sqlserver.md)
 
 R2DBC:
-* [Connecting to MySQL using R2DBC](docs/r2dbc-mysql.md)
-* [Connecting to Postgres using R2DBC](docs/r2dbc-postgres.md)
-* [Connecting to SQL Server using R2DBC](docs/r2dbc-sqlserver.md)
+* [Connecting to Cloud SQL using R2DBC](docs/r2dbc)
 
 For examples of this library being used in the context of an application, check out the sample applications located 
 [here](https://github.com/GoogleCloudPlatform/java-docs-samples/tree/master/cloud-sql).