Restructure execution mode pages

[jyu00]

URL to the relevant documentation

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• typo
• code bug
• out-of-date content
• broken link
• other

Describe the fix.

While reviewing #2105, I had some ideas on how we can restructure the execution modes pages. I'm opening a new issue instead of commenting in the PR in case we want to merge the PR first.

Introduction to execution modes

1. First introduce what execution modes are. The one liner we have today "There are several ways to run workloads", IMHO, isn't enough introduction. It should instead mention what execution modes and why one should care. For example, the execution modes indicate how your jobs are to be scheduled, and choosing the right execution mode allows your jobs to run efficiently within the time budget you have.
2. Then it can go into a brief introduction on what modes we have and what each does
3. Then the concepts of active, max, and iTTL. These apply to both session and batch. Much of the existing "Introduction to session" can merged in here.

Choosing the right execution mode

1. Some introduction on things to consider, such as usage, overall execution time, iTTL requirement, and time in between jobs
2. Explain what usage means (e.g. it reduces your priority)
3. Give recommendation, including best practices
4. Give some examples - e.g. what should you expect when running an iterative workload using session vs batch vs job

Run jobs in a session/batch

These pages would focus on code examples. Things like why you should use batch, which is currently mentioned in the Run.. page, should be moved to the Choosing... page.