Fix migrating alerts aliases (#74898)

* Add comments to indicate which page I believe each alias is intending to redirect

Signed-off-by: Jack Baldry <jack.baldry@grafana.com>

* Remove difference-old-new redirect from migrating-alerts page since an alternative page exists

The alternative page is `docs/sources/alerting/difference-old-new.md`.

Signed-off-by: Jack Baldry <jack.baldry@grafana.com>

* Add comment to show intended absolute path redirect

Signed-off-by: Jack Baldry <jack.baldry@grafana.com>

* Remove `unified-alerting/difference-old-new/` redirect because it already exists in `docs/sources/alerting/difference-old-new.md`.

Signed-off-by: Jack Baldry <jack.baldry@grafana.com>

* Remove `unified-alerting/` alias since the absolute URL already redirects to `/docs/grafana/latest/alerting/`

Signed-off-by: Jack Baldry <jack.baldry@grafana.com>

* Fix `/docs/grafana/<GRAFANA VERSION>/alerting/migrating-alerts/` redirect

The following example uses `latest` as _`GRAFANA VERSION`_ for simplicity and readability.

The desire is the following redirect:

```
src: /docs/grafana/latest/alerting/migrating-alerts/
dst: /docs/grafana/latest/alerting/set-up/migrating-alerts/
```

`dst` is the Hugo pretty URL for the current page (`docs/sources/alerting/set-up/migrating-alerts/_index.md`).

When constructing Hugo aliases we are working from the page's containing directory and not the page itself.
The path element `.` represents that directory.
For example:

```
 page: /docs/grafana/latest/alerting/set-up/migrating-alerts/
alias: ./
  dst: /docs/grafana/latest/alerting/set-up/
```

The path element `..` represents the parent directory of the page's containing directory.
For example:

```
 page: /docs/grafana/latest/alerting/set-up/migrating-alerts/
alias: ../
  dst: /docs/grafana/latest/alerting/
```

Signed-off-by: Jack Baldry <jack.baldry@grafana.com>

---------

Signed-off-by: Jack Baldry <jack.baldry@grafana.com>

docs/sources/alerting/difference-old-new.md

@@ -2,7 +2,7 @@
 _build:
   list: false
 aliases:
-  - unified-alerting/difference-old-new/
+  - ./unified-alerting/difference-old-new/ # /docs/grafana/<GRAFANA VERSION>/alerting/unified-alerting/difference-old-new/
 canonical: https://grafana.com/docs/grafana/latest/alerting/difference-old-new/
 description: Compare new unified alerting compared to legacy dashboard alerting
 keywords:

docs/sources/alerting/set-up/migrating-alerts/_index.md

@@ -1,9 +1,6 @@
 ---
 aliases:
-  - difference-old-new/
-  - unified-alerting/
-  - unified-alerting/difference-old-new/
-  - alerting/migrating-alerts/
+  - ../migrating-alerts/ # /docs/grafana/<GRAFANA VERSION>/alerting/migrating-alerts/
 canonical: https://grafana.com/docs/grafana/latest/alerting/set-up/migrating-alerts/
 description: Upgrade Grafana alerts
 labels: