guide: Running Experiments improvements

[jorgeorpinel]

Follow up to #2690

• The pipeline section seems too deep. In fact (updated in ref: extract technical details from exp run to guides #3182)
  removing the pipeline and stages concept would be ideal. I think the guide could link to other parts of the docs more liberally.
• The text under (Hyper)parameters has nothing to do with running experiments initially. (ref: extract technical details from exp run to guides #3182)
• Move exp implementation details to new Overvie page? (guide: DVC Experiments Overview #2909)
• Mention exp IDs and names and/or at least cover --name (see [NESTED] guide: Exp implementation details, naming into Overview #3006 (review)).
• Add info about metrics?
• Explain that experiments can also track changes to data, code, etc.
• Delete note on "each experiment runs in an independent temporary directory" ? Has been hidden
• Review conceptual level of each section e.g. H2s for main subtopics, H3s for methods, hidden blocks for implementation <details>
• Checkpoint experiments is basically pulled from https://dvc.org/doc/command-reference/exp/run#checkpoints. We need to determine what to leave on the cmd ref. (rel 32548)
• exp run --reset : Clarify behavior #2745
• Maybe also ref: exp run --set-param with templating #2313
• Combine with Hydra composition? (https://github.com/iterative/dvc.org/pull/4041/files#r994078998)
• Document exp run param overrides and grid search (Hydra feature). Context

[iesahin]

I believe the command reference should contain examples with one or two sentence explanations and the rest should be moved to UG. Command examples can continue to live in the ref. WDYT @jorgeorpinel

[jorgeorpinel]

@iesahin Yes, we need some cmd ref examples focused on how to use the CLI (not so much on explaining concepts). There can be examples in both places.