Update get.asciidoc

Minor improvements.

curl -XHEAD doesn't actually print anything so I've changed to use -I which actually prints the headers received.
[DOCS] improved get docs

- added _version to response
- exists call use -XHEAD with -i flag to include headers in the output
Make sure that parent type is created explicitly.
Cleaned up the p/c integration tests to use shorter mapping definition, ensureGreen() and .get()
Fixed issue where highlighting in percolate existing doc api doesn't work (no highlight snippets)