Add a search page to show all results of a search in a single page

[cderv]

Idea from @apreshill to have something similar to jupyterbook: https://jupyterbook.org/search.html?q=notebook

This would also allow sharing a URL of search result (#1064)

Currently search result are show in a new dynamic window in bs4_book() which is already a better experience than in gitbook()

Related to #1031