Update encrypted-hiera-data doc
Update encrypted-hiera-data doc

I've added some more details in places and removed mentions of the VM as
this has now pretty much been retired.

All looks fine by me reading though, i've tested the basic stuff in this
doc like decrypting a file and re-encrypting etc.

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