Renamed doc_collections to just-the-docs.collections

- font size 16px to prevent zooming on iOS Safari
- search input is scrolled into view on iOS Safari
- box-shadow is displayed correctly

Added multiple previews, configurable by site.search.previews
Improved search-data.json

Improved documentation for search configurations

Added more search configurations
Display "no results found"

This reverts commit 50cdf076.

The default is for hyphens to separate tokens in search terms: `gem-based` is equivalent to `gem based`.

This adds `search_tokenizer_separator` as a site configuation option, to support search for hyphenated words.

(cherry picked from commit 99371e971406090a2dba7b9b69f514a8f87afce3)

(cherry picked from commit 468264941d08b00252a4f6f081bc2dff5920bffe)

(cherry picked from commit 9f3f38acf4e1ac40e3fb2b2f86018d4cba28dd07)

(cherry picked from commit 86ff2dac05412a6ff994968c2be6122a7fcf85f9)

(cherry picked from commit d39b7b3041d1bdb9f80ab7f5878e64a3b3b79b5d)

(cherry picked from commit f25d76730b175fb680aa2770fb20a67fd858ffbb)

(cherry picked from commit db50f282bab1e98397cc7434a811b8b7da2fe6dc)

(cherry picked from commit 2b153ed074811e735aaa58d57a3fcdf7838e17f5)

(cherry picked from commit dffb2706a158784e2f3091f895a868e373683bc8)