start: improvements

[flippedcoder]

• Remove the Sharing experiments section from Get Started > Experiments

We should remove the Sharing experiments section since it adds more complexity to the Get Started tutorial than there needs to be since users have to set up a Git remote as well as a DVC remote. We don't go over this at any other point in the tutorial.

If a user clones the get started repo, it works fine, but if they are following along there are some roadblocks they might run into with getting things set up. For example, I ran into a weird credentials issue with my Git remote and ended up working with @dberenbaum to do a crazy workaround.

Overall, it clouds up the purpose of this section which is running experiments.

• Update the git add command for Get Started > Metrics, Parameters, and Plots

In the Comparing iterations section, users can get empty Old and Change values when they run dvc metrics diff because the dvc.lock and dvc.yaml are missing from the git add.

We need to update the command to be git add dvc.lock dvc.yaml scores.json prc.json roc.json.

[jorgeorpinel]

May also be good to see if we can consolidate with #1943.