chore(docs): docs app initialization performance

## Stage 1 - delay search index build for one second
Following in the best traditions of agile development. Here is a minimal improvement in the user experience when they appear at the docs website. The page now renders immediately but then locks the page after 1 sec while it builds the index. This is a slight UX improvement. The next stage is to move this processing to a WebWorker to prevent it blocking the main thread.

Related to #9203