Elasticsearch developer documentation

Suggested by @smcgivern

GitLab's elasticsearch integration is fairly difficult to get to terms with, and we don't have as much developer expertise with it as we'd like.

To address this, we should put together some developer documentation, detailing how it works in a reasonably detailed manner, in doc/development/.... This will help developers new to the space to contribute.

/cc @smcgivern @vsizov