[TEP-0044] Update Alternatives

This commit describes design constraints related to running multiple Tasks in one pod in more detail.
It also updates the alternative solutions based on the clarified problem statement.

Changes:
- Add 3 options: pipeline in a pod + pipelines in pipelines, pipeline -> taskrun, and task pre and post steps
- Modify and add more detail to the "TaskGroup" proposal
- Remove "focus on workspaces" option: this adds new syntax but not new functionality
- Remove "custom pipeline" solution due to similarity with grouping CRD solution
- Remove "custom scheduler" option: This is a solution for scheduling parallel Tasks that share a PVC
  on the same node, but does not address the problems of pod overhead and the need
  to share data between tasks in the first place

Co-authored-by: Christie Wilson [email protected]