checkout example is simplified by using fetch

[imhardikj]

This PR updates \content\docs\commands-reference\checkout.md to fix #528 (comment).
checkout example is simplified by using 'dvc fetch`.

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Some specific subtasks:

• cmd ref: use example-get-started repo in pull examples #528 (comment):

  dvc fetch could be hidden in the "Click and expand to setup the project"...

• cmd ref: use example-get-started repo in pull examples #528 (comment):

  fetch -aT is not currently used in the expandable section. The suggestion is to add it there so it can be removed from the block further down. There's an explanation about it under that block too, which would probably need to go away (moved into the expandable section too).

• &

  However the fetch + checkout parenthesis would have to go away too (not moved, just gone). Instead maybe we can just link "fetch" and "checkout" in We can now just run dvc pull that will fetch and checkout the...

[jorgeorpinel]

Great stuff so far, thanks! Needs a little bit of polishing details.

[jorgeorpinel]

Hi. I noticed you pushed a commit @imhardikj is this ready to re-review? Please request so when ready from the top of the PR:

[imhardikj]

@jorgeorpinel I missed to make a review request. If the changes in checkout are correct can I proceed to work on pull example.

[jorgeorpinel]

Just one more suggestion but otherwise this is good!