resources/page: Use binary search in Pages.Prev/Next if possible

This is obviously much faster for lager data sets:

```bash
name                         old time/op    new time/op    delta
SearchPage/ByWeight-100-4       267ns ± 4%     272ns ± 5%     ~     (p=0.457 n=4+4)
SearchPage/ByWeight-5000-4     10.8µs ± 3%     1.2µs ± 2%  -88.99%  (p=0.029 n=4+4)
SearchPage/ByWeight-10000-4    21.1µs ± 1%     1.4µs ±11%  -93.28%  (p=0.029 n=4+4)
```

See #4500

resources/page/pages_sort_test.go

@@ -269,6 +269,7 @@ func createSortTestPages(num int) Pages {
 	for i := 0; i < num; i++ {
 		p := newTestPage()
 		p.path = fmt.Sprintf("/x/y/p%d.md", i)
+		p.title = fmt.Sprintf("Title %d", i%(num+1/2))
 		p.params = map[string]interface{}{
 			"arbitrarily": map[string]interface{}{
 				"nested": ("xyz" + fmt.Sprintf("%v", 100-i)),