std docs: enhance search browser history UX
std docs: enhance search browser history UX

Before this change every keypress in the search field causes a browser
history entry, which makes navigating back annoying.

On first keypress in the search field, a new history entry is created.
On subsequent keypresses, the most recent history entry is replaced.
Therefore a typical history after searching and navigating to an entry
might look like

1. documentation root
2. search page "print"
3. docs for `std.debug.print`


Co-authored-by: Žiga Željko <74d5b0a09b4c07261d331a53946620c92a658bd3@gmail.com>

zig-lang/zig,andrewrk/zig,andrewrk/zig,zig-lang/zig,zig-lang/zig