Update documents for migrated extensions

1. Update README for migrated extensions.The newly migrated extensions modules have suffix -ext in the artifact name.Review and update extensions README docs to correct artifact name. An example for correct artifact name would be ignite-flink-ext
2.  We need to update some sections (maven artifacts name, a paragraph about the extensions repository) in existing docs before the extension is released: https://github.com/apache/ignite/tree/master/docs/_docs/extensions-and-integrations

3. As for the extensions web page, I see its structure as follows. It goeswith a header introducing the extensions repository and covering types ofcontributions that are welcomed there. Then down below in the body we listall the available extensions - it can be a simple table with an extensionname, description, URL or a bit more advanced interface with a grid layoutlike on this page [1]. With the grid layout, we can add logos of theprojects Ignite is integrated with in addition to the name/description/URL.Let me know what you think.[1] https://ignite.apache.org/screencasts.html

http://apache-ignite-developers.2346864.n4.nabble.com/IGNITE-12358-Migrate-ZeroMQ-module-to-ignite-extensions-td45067.html#a46863