Make docs search results URL-addressable

This change adds two improvements to docs searching functionality.

First, search results will immediately be displayed when a ?search=searchterm
query string parameter is provided to any docs url.

Second, search results are now inserted into the browser history, allowing for
easier navigation between search results and docs pages.
auto merge of #11760 : dmac/rust/addressable-search, r=alexcrichton

This change adds two improvements to docs searching functionality.

First, search results will immediately be displayed when a ?search=searchterm
query string parameter is provided to any docs url.

Second, search results are now inserted into the browser history, allowing for
easier navigation between search results and docs pages.