Rewrite the varnish cookbook article

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #1505, #4176
is it ok to just throw out the purge information here and reference FOSHttpCacheBundle instead?

the x-forwarded-proto moved to the top as its actually always required to get a correct setup