ENH: create a navbar_persistent section

[12rambau]

Fix #1006
May superceed #1020

Instead of adding complex CSS rules to make sure the search button cannot be displayed with the search bar this PR simply adds an extra area in the navbar: navbar_persistentthat will do exactly what the current search button is doing now.

I also adapted the documentation accordingly

to make the search-button disappear:

html_theme_options: {
    #...
    "navbar_persistent": [],
    #...
}

[drammock]

this LGTM, much better than the approach in #1020.