[Feature/Documentation request] Add documentation for tracking dependencies.

#### To request a feature...

* Clearly describe the feature.

On the [docs page](https://taskwarrior.org/docs/) describe task dependencies.

I have never used taskwarrior and am browsing the documentation to determine if it's the best tool for me. I couldn't find information on how to manage dependencies after reading the docs page + FAQ page for about 10 minutes.

I read all of Introduction, Commandline Syntax, 30 Second Tutorial. I then searched the FAQ and issue tracker for "dependencies".

I found a single FAQ entry mentioning "dependencies" yet I can't evaluate what the answer means without better context. I searched "dependency" in this github issue tracker and saw multiple issues that make it seems like some support for dependencies are available.

After using "find in page" I see some intro in "Best Practices". It would help me to have a separate entry to describe how dependencies work, or to mention dependencies are a supported feature on one of the front pages.

* Clearly state the use case. We are only interested in use cases, do not waste time with implementation details or suggested syntax.

The use case for documentation is so that new users can quickly learn how to manage dependency relationships between tasks.

* Please see our notes on [How to request a feature](https://taskwarrior.org/docs/features.html)

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