Add documentation of node classes with single instances
Add documentation of node classes with single instances

The node classes documented here are assumed to be non-critical for the operation of
GOV.UK and not highly available. This means that developers looking to
implement critical services should avoid to rely on these instance types.

@schmie

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