Update "GOV.UK in AWS".
Update "GOV.UK in AWS".

* Mention the move from self-hosted MongoDB to DocumentDB.
* email-alert-api and publishing-api aren't on separate RDS instances.
* Update a very outdated link to hieradata for which apps run where.

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