Update documentation README/contribution guide

As a framework contributor, I want to be able to run the antora documentation locally to verify changes before adding PRs.

### Suggested Documentation change

* All links in the opening section are broken (404)
* The link to the `[AxonIQ Library Contribution Guide](https://docs.axoniq.io/contribution_guide/index.html)` is broken
* The link to the `[detailed information on how the process to build the docs works](https://docs.axoniq.io/contribution_guide/overview/build.html)` is broken
* Point 1: _Make sure you have Node (a LTS version is preferred), Antora and Vale installed in your system._ -> only node and vale need to be provided, antora-cli comes via npm install/`package.json`

Note: We also discussed wether a custom README in the axon-framework docs is required, as antora does a multi-repo doc and there could be just one single README how to use it.

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