Add search bar

[kimmcintyre]

Addresses issue #91

Turns on DocFX's search bar for the doc site. Here is an example of searching for DepthStencilState in my local doc site.

DocFX's search bar is powered by lunr.js and therefore is subject to the idiosyncrasies of this search engine as described in the lunr docs.

For example, lunr assumes each term in the search query is atomic. So searching for graph and graphic will yield different results.

Additionally, lunr assumes a logical OR when searching with multiple terms. Therefore, searching aspect ratio will return the combined results of the two separate search queries for aspect and ratio but not for the specific phrase aspect ratio. This can be partially mitigated by using the search query +aspect +ratio but this will only serve to return documents that have both of these words but not necessarily together in the phrase aspect ratio

So the search functionality that DocFX provides is not perfect, but I do believe that the additional functionality it has over the ToC filter makes it worth enabling.