search: add a link to the search howto page

This adds a link from the site's search widget to a guide which
demonstrates the different search operators that the reader can use to
narrow down their search results.

The link is visible on /search/, as well as any page that embeds the
search widget (such as /docs/howto/). It is not visible in the page
header's search widget (because I'm not yet sure how to make that
change), but an equivalent link could probably be added to that popup
element if this link proves to be useful to users.

Also: remove a pointless redirect when the "search" link is used from
the site's 404 page.

Closes cue-lang/docs-and-content#1.

Preview-Path: /search/
Preview-Path: /docs/howto/
Signed-off-by: Jonathan Matthews <[email protected]>
Change-Id: Ibe287f58295320141448574a1b91f8bd9ec00b14
Dispatch-Trailer: {"type":"trybot","CL":1206957,"patchset":2,"ref":"refs/changes/57/1206957/2","targetBranch":"master"}

hugo/i18n/en.toml

@@ -87,7 +87,7 @@ other = """
 Oops! This page doesn't exist.<br><br>
 Try going back to our <strong><a href="/">home page</a></strong>,
 taking a look at our <strong><a href="/docs/">documentation</a></strong>,
-or <a href="/search">searching for a page</a></strong>.
+or <a href="/search/">searching for a page</a></strong>.
 """
 
 # Search
@@ -99,6 +99,8 @@ other = "What are you looking for?"
 other = "documents found"
 [search_filters_title]
 other = "Filters"
+[search_help_link_text]
+other = "How can I customize this search?"
 [search_placeholder]
 other = "Search in cuelang.org"
 [search_placeholder_content_type]

hugo/layouts/partials/search.html

@@ -40,6 +40,7 @@
             </div>
 
             <div class="search__filters">
+                <p class="search__filters-title"> <a href="/docs/howto/search-this-site/">{{ T "search_help_link_text" }}</a></p>
                 <p class="search__filters-title"> {{ T "search_filters_title" }}</p>
 
                 {{- partial "filter.html" (dict