Move restore for source install below others

What does this MR do?

Moves "restore from backup" instructions for Source installations below the others. Omnibus and Docker/Helm installations account for the vast majority of GitLab instances, so it makes sense to have them higher in the visual hierarchy.

Does this MR meet the acceptance criteria?

Yes, I think so!

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.