Contributing docs omit many steps

So far I've noticed the following packages and tools that are not available by default on a Debian system, need to be installed to build and test beam, and are not yet mentioned in all our docs:

* tox
* Go
* Python (which versions?)
* Docker

There might be more. The docs such as README.md should be updated to include these. This covers README.md, CONTRIBUTING.md, https://cwiki.apache.org/confluence/display/BEAM/Contributor****FAQ, and https://beam.apache.org/contribute/

That we have four such docs is itself an issue. We should probably consolidate these.

Imported from Jira [BEAM-8798](https://issues.apache.org/jira/browse/BEAM-8798). Original Jira may contain additional context.
Reported by: elharo.