Refactor Gradle article

[mshabarov]

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The article about Gradle https://vaadin.com/docs/latest/getting-started/start/alternatives/gradle is too hidden from reader and has too many content for a "get started" article.

I propose to move the article with the whole content next to https://vaadin.com/docs/latest/flow/configuration/maven article (same level), and leave only "get started" and general information in https://vaadin.com/docs/latest/getting-started/start/alternatives/gradle.

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