[docs] pipeline execution concept page 1/

Summary:
https://github.com/dagster-io/dagster/issues/3964
move old content back:

• ways to execute pipelines
• solid selection + add more dagit screenshots https://github.com/dagster-io/dagster/issues/3896

next:

• more examples

Test Plan: https://dagster-git-yuhan-docs-execution-elementl.vercel.app/master/concepts/solids-pipelines/pipeline-execution

Reviewers: sashank, alangenfeld, sandyryza

Reviewed By: sashank

Differential Revision: https://dagster.phacility.com/D8547