Documentation for Reactive programming

[tsegismont]

In various docs we have a section for Reactive programming with RxJava 2.

For example:

• https://vertx.io/docs/vertx-cassandra-client/java/#_rxjava_2_api
• https://vertx.io/docs/vertx-pg-client/java/#_rxjava_2_api

Given RxJava 2 is no longer maintained, I think we should update those to RxJava 3.

In most cases it consists in changing titles/wording, not examples themselves.

• [x] vertx-rx https://github.com/vert-x3/vertx-rx/pull/264
• [x] vertx-cassandra-client https://github.com/vert-x3/vertx-cassandra-client/pull/73
• [x] vertx-kafka-client https://github.com/vert-x3/vertx-kafka-client/pull/204
• [x] vertx-mongo-client https://github.com/vert-x3/vertx-mongo-client/pull/258
• [x] vertx-pg-client, vertx-sql-common https://github.com/eclipse-vertx/vertx-sql-client/pull/973
• [x] vertx-web-api-contract, vertx-web-client, vertx-web-graphql https://github.com/vert-x3/vertx-web/pull/1967
• [x] vertx-junit5 https://github.com/vert-x3/vertx-junit5/pull/105
• [x] vertx-lang-kotlin-coroutines https://github.com/vert-x3/vertx-lang-kotlin/pull/201
• [x] vertx-service-discovery https://github.com/vert-x3/vertx-service-discovery/pull/150

[tsegismont]

@vietj I built vertx-stack/stack-docs and checked the changes. Tell me if you want to review the PRs or if I shall go ahead and merge.

[vietj]

LGTM