Split and update architecture diagrams

[pianohacker]

This pull request aims to:

• update the architecture diagrams and explanatory text to be up to date with the latest log/metric architecture
• split the architecture diagram into the Loggregator path and the shared-nothing path, to make the diagrams slightly more comprehensible
• update the styling of the diagrams

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this. Unfortunately, the Pivotal Tracker project is private so you may be unable to view the contents of the story.

The labels on this github issue will be updated when the story is started.

[mlimonczenko]

Thanks so much, @pianohacker!