Make the ddox instant search available for ddoc

[dlangBugzillaToGithub]

Seb reported this on 2018-03-19T23:22:56Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=18636

Description

this is an common request from the "State of D" survey.

Sometimes the Rust instant search was a good example.
The main difference here is that the entire page is used for listing the results.