user-guide: write about managing metrics

[efiop]

A small article on how to use dvc metrics to manage metrics for your project in the most convenient way.

[shcheklein]

@efiop What should we highlight for the "most convenient" way? Any hints, tricks on top of what we already have in get started?

[efiop]

The current example in 'get-started' is too simple and too specific. I was talking about a more broad article with tips such as repro -m.

[shcheklein]

Would be beneficial to mention about repro -m in the dvc metrics. It can go to the #72 as well.

[shcheklein]

I think we can close this - we have experiments sections in the Get Started, we have index page in mertics and plots. I think we can do use case about tracking/managing experiments, but it's a separate task. I don't think we need a user guide page for this at the moment.