docs: managing runs

[guenthermi]

docs: managing runs

• Explains the concept of a run in Finetuner

• Explain all the attributes and functions of the Run object in detail or links to other places where they are documented

• Mention ways to view runs in JAC

• [ ] This PR references an open issue

• [x] I have added a line about this change to CHANGELOG

[github-actions[bot]]

:memo: Docs are deployed on https://ft-docs-managing-runs--jina-docs.netlify.app/docs-managing-runs :tada: