Review the structure and flow of Planning Foreman

[aneta-petrova]

During the review of #4210, a few concerns regarding the current structure, flow and generally the idea behind the Planning guide were raised.

• Starting with a description of content management can be too much for users (Katello builds). See https://github.com/theforeman/foreman-documentation/pull/4210/files#r2355280797 Possible solution: Start with describing the major components, and then move to content and provisioning management?

• Reordering of the deployment-themed chapters and concept/considerations chapters was suggested. See https://github.com/theforeman/foreman-documentation/pull/4210/files#r2355306689

The team should discuss these suggestions and look at the Planning guide to see what improvements could potentially be made.