Update MongoDB backup documentation
Update MongoDB backup documentation

* Tighten language
* Fix broken link to another manual page
* Replace Amazon with AWS for precision
* Create "Restoring" sections to focus reader on relevant content

I have tested the page locally and it looks OK.

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs