Fix some server rebuild issues for non-HTML custom output formats

The failing test case here is

* A custom search output format defined on the home page, marked as `noAlternative` and not `permalinkable`
* In fast render mode, when making a change to a data source for that search output format, the JSON file was not refreshed.

There are variants of the above, but the gist of it is:

* The change set was correctly determined, but since the search JSON file was not in the recently visited browser stack, we skipped rendering it.

Running with `hugo server --disableFastRender` would be a workaround for the above.

This commit fixes this by:

* Adding a check for the HTTP request header `Sec-Fetch-Mode = navigation` to the condition for if we should track server request as a user navigation (and not e.g. a HTTP request for a linked CSS stylesheet).
* Making sure that we compare against the real relative URL for non-permalinkable output formats.

Fixes #13014

hugolib/page__paths.go

@@ -71,11 +71,12 @@ func newPagePaths(ps *pageState) (pagePaths, error) {
 		// Use the main format for permalinks, usually HTML.
 		permalinksIndex := 0
 		if f.Permalinkable {
-			// Unless it's permalinkable
+			// Unless it's permalinkable.
 			permalinksIndex = i
 		}
 
 		targets[f.Name] = targetPathsHolder{
+			relURL:       relPermalink,
 			paths:        paths,
 			OutputFormat: pageOutputFormats[permalinksIndex],
 		}