Improve 'Environment data sync' documentation
Improve 'Environment data sync' documentation

Summary of changes:

- Moved link to govuk_env_sync to top of document. Previously the
  first link was to env-sync-and-backup, so scanning the page and
  clicking the first env-related link brought you to outdated code.
- Summarise what it does first, followed by how that works under
  the hood.
- Moved 'data sanitisation' to its own subsection, as it's an
  interesting and unexpected feature to an outsider.
- Link to the relevant bits in govuk-puppet under the 'How it
  works' section, as otherwise it's all a bit theoretical.
- Moved Gor section up to the data sync times section, as they're
  closely connected. Also updated it to 'GoReplay' as per the
  document it references.

I resisted the temptation to remove the example configuration
and/or the 'meanings of parameters', as someone presumably sees
value in this. I do think it duplicates the code and code comments
though, and we'd be better off just pointing to that.

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