Rewrite the varnish cookbook article

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #1505, #4176
> Do we have any data about the market share of each Varnish version?

no idea, but maybe the internet has:
https://twitter.com/dbu/status/544502440041922561

but i tend to drop it. its really outdated so new projects hopefully go
with 3 - or will need to get used to find only documentation for newer
versions anyways.