v4 - Improve Docs Search

[surjithctly]

The current search in v4 docs only looking for Page Titles, not the actual content. Due to the structure change and ease of usability, I recommend improving the search to all content and code. So it will be easier to look for a specific element.

There are some search providers like Agolia and Swiftype. I hope it will be easier to integrate those.

On version 3, I was (most of the people may be) using the Browser Search CTRL + F. Since in new version, all sections are different page, its impossible to do that too :(

[bardiharborow]

Duplicate of #21125.