vmalert: [bug] fixed System hyperlink 404 redirect (#6620)

### Describe Your Changes

As mentioned in https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6603, some hyperlinks under `vmalert` -> `System`
section is not working as expected.

Pages and redirection:
- For page `http://127.0.0.1:8880/`: `flags` button will redirect to
`http://127.0.0.1:8880/flags`
- For page `http://127.0.0.1:8880/vmalert`:
`http://127.0.0.1:8880/flags`
- For page `http://127.0.0.1:8880/vmalert/`:
`http://127.0.0.1:8880/vmalert/flags` (page not exists)
- Similar redirection could be observed with `-http.pathPrefix`

Two potential ways to avoid 404 redirection:
1. **avoid visiting `/vmalert/`** (I'm trying to do this).
2. provide support for `/vmalert/flags`.

`/vmalert/` could be visit only when user click other navigator (e.g.
Group) and click vmalert again:
![Peek 2024-07-10
10-07](https://github.com/VictoriaMetrics/VictoriaMetrics/assets/30280396/13d7b147-a1b6-4e93-9ee0-26f881a16bef)
Because: `http://127.0.0.1:8880/vmalert/groups?search=` + `<a
class="nav-link" href=".">` = `http://127.0.0.1:8880/vmalert/`

So I'm trying to change the `href="."` to `href="../vmalert"`.

### Checklist

The following checks are **mandatory**:

- [X] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).

app/vmalert/web.go

@@ -35,7 +35,7 @@ var (
 		{"-/reload", "reload configuration"},
 	}
 	navItems = []tpl.NavItem{
-		{Name: "vmalert", Url: "."},
+		{Name: "vmalert", Url: "../vmalert"},
 		{Name: "Groups", Url: "groups"},
 		{Name: "Alerts", Url: "alerts"},
 		{Name: "Notifiers", Url: "notifiers"},

docs/CHANGELOG.md

@@ -57,6 +57,7 @@ See also [LTS releases](https://docs.victoriametrics.com/lts-releases/).
 * BUGFIX: [docker-compose](https://github.com/VictoriaMetrics/VictoriaMetrics/tree/master/deployment/docker#docker-compose-environment-for-victoriametrics): fix incorrect link to vmui from [VictoriaMetrics plugin in Grafana](https://github.com/VictoriaMetrics/VictoriaMetrics/tree/master/deployment/docker#grafana).
 * BUGFIX: [docker-compose](https://github.com/VictoriaMetrics/VictoriaMetrics/tree/master/deployment/docker#docker-compose-environment-for-victoriametrics): fix incorrect link to vmui from [VictoriaMetrics plugin in Grafana](https://github.com/VictoriaMetrics/VictoriaMetrics/tree/master/deployment/docker#grafana).
 * BUGFIX: [Single-node VictoriaMetrics](https://docs.victoriametrics.com/) and `vmstorage` in [VictoriaMetrics cluster](https://docs.victoriametrics.com/cluster-victoriametrics/): Fix the dateMetricIDCache consistency issue that leads to duplicate per-day index entries when new time series are inserted concurrently. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6534) for details.
+* BUGFIX: [vmalert](https://docs.victoriametrics.com/vmalert/): fix incorrect redirection in WebUI of vmalert. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6603) and [this pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/6620).
 * BUGFIX: [vmui](https://docs.victoriametrics.com/#vmui): fix input cursor position reset in modal settings. See [this pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/6530).
 * BUGFIX: [vmbackupmanager](https://docs.victoriametrics.com/vmbackupmanager/): fix `vm_backup_last_run_failed` metric not being properly initialized during startup. Previously, it could imply an error even if the backup have been completed successfully. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6550) for the details.
 * BUGFIX: [MetricsQL](https://docs.victoriametrics.com/MetricsQL.html): properly calculate [histogram_quantile](https://docs.victoriametrics.com/MetricsQL.html#histogram_quantile) over Prometheus buckets with inconsistent values. See [this comment](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4580#issuecomment-2186659102) and [this pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/6547). Updates [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2819).