cmd ref: review `root` ref to be aligned with other commands

[shcheklein]

See https://dvc.org/doc/commands-reference/root.

i.

• The layout of this command reference is outdated.

• It should be the same as for the cache for example.

ii.

• Description should include a better motivation for this command - mostly ease of generating stage files no matter where you run the dvc run.

• Examples should be actionable and meaningful. It's should be easy to run commands to see the results.

iii.

• A new example about shell scripts to generate dvc.yaml files is needed to complete this update. See Update dvc root command reference motivation and examples #1637 (comment) Now part of doc: change run mentions to dvc.yaml or stage add + repro #2076

[vibhor98]

Hi @shcheklein, I would like to take up this issue. Thanks!

[shcheklein]

Partially done by @vibhor98 . I've updated the description a little considered the changes done.

[jeremydesroches]

@jorgeorpinel I’ll work on closing this out.