Plan content improvement for Assisted Installer

User guide needs tighter organization in general.

• UI installation section is bloated but functional.
• API installation section is critical.
• Other sections are lower priority but need to be moved around to reduce repetition and create a better flow:
  • Optional: Configuring schedulable control plane nodes
  • Optional: Enabling disk encryption
  • Configuring discovery image, Preinstallation validations, Expanding cluster....

If the organization is improved, the numerous xrefs and multiple Additional resource sections could be reduced.

See Miro board for details.