mark search as disabled for now

[zkamvar]

I thought I opened an issue to add functionality to the search button, but it turns out that I only generated a commit that begins the process with a note about what I found:

While Bootstrap 5 may have search enabled through their forms, it turns
out that it's difficult for me to find documentation for this specific
task. Moreover, the pkgdown searchbar never had a button associated with
the search... and for good reason: the search is dynamic; it does not
bring you to a page with your search results; it gives you the search
results as a dropdown menu, so you go to your result by clicking
"enter". It makes sense! I think we can provide a blank page for search
results, but we will need to test out the accessibility for sure.

The steps for me to resolve this are:

• add "disabled" attribute to the search form to clearly indicate that it is not available for input
• add pkgdown::build_search() after all the pages are built in sandpaper:::build_site()
• decide if a search button is needed or if the form output will be sufficient (this is where I might need some help with screenreader testing from @brynnelliott)
• implement the search form JavaScript and CSS in {varnish}

Originally posted by @zkamvar in carpentries/workbench#8 (comment)