MG-11 - Remove super admin content from billing

[wambui-pixel]

Pull request title should be MG-XXX - description or NOISSUE - description where XXX is ID of issue that this PR relate to.
Please review the CONTRIBUTING.md file for detailed contributing guidelines.

What does this do?

• Removes super admin content from user's guide.
• Adds an introduction to domain management section.
• Updates invitations section.
• Updates domain section.
• Removes unused images

Which issue(s) does this PR fix/relate to?

Resolves #11
Put here Resolves #XXX to auto-close the issue that your PR fixes (if such)

List any changes that modify/break current functionality

n/a

Have you included tests for your changes?

n/a

Did you document any new/modified functionality?

n/a

Notes

[ianmuchyri]

• We can't edit the id

• Not all fields can be edited in the drop down. Move this section below the first image then tell the user how to get there
  

• There is no flow to this sentence
  

• syntax
  

• We currently only support card payments
  

• use back ticks