Update content for workflows core concept page

[Aviatorscode2]

Summary

Adds the "Workflow" core concept page to the documentation. This page explains workflows as the central orchestration layer in Novu, describing their structure, execution, and configuration.

Key Highlights

• Defines what a workflow is and how it functions within Novu’s architecture.
• Covers the Workflow Editor, notification templates, step types, and filters.
• Documents triggering mechanics (including payloads, subscribers, and job lifecycle).
• Explains workflow status, tags, and identifiers.
• Includes advanced features like subscriber preferences, critical workflows, and conditional logic.

Purpose

Provides a complete conceptual foundation for understanding and using workflows in Novu.

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[DianaHackmamba]

Great edit of this content, however, this is waaaay to long for a user to read through.

• Break this up into smaller topics.
• Figure out if some of the content should go elsewhere. For example, Triggering workflows is definitely a task. Explaining how to view and filter the activity feed is also a task.

Also think about where content should actually be. For example, the Channel Steps topic in Building Workflows is pretty sparse, so the explanations here might fit better there. Same with basically everything else in there, including the Overview, which is where most of this content should probably be. Possibly all of it.

A concept should provide a high-level explanation of what something is and what you can do with it. Right now, there is too much detail and explanation.

[DianaHackmamba]

Re-request review when you are done making changes. Also, please add a description to this PR.

[DianaHackmamba]

One very minor update, then G2G.