Improve documentation for more restrictive api key use

I've added more details so only the least required amount of privileges are given to use this widget and also removed the `uptime` field which is non-functional and [I can't see a reference to it in the code](https://github.com/benphelps/homepage/blob/main/src/widgets/opnsense/component.jsx#L23-L26), so I think it was a leftover from a conversation [here.](gethomepage/homepage#730 (comment))

src/pages/en/services/opnsense.md

@@ -4,16 +4,16 @@ description: OPNSense Widget Configuration
 layout: ../../../layouts/MainLayout.astro
 ---
 
-API key / secret can be generated via the admin, see [the opnsense docs](https://docs.opnsense.org/development/how-tos/api.html#creating-keys).
+The API key & secret can be generated via the webui, by creating a new user at `System/Access/Users` called `homepage-api`, ensuring `Generate a scrambled password to prevent local database logins for this user.` is checked, and then edit the effective privileges selecting `Diagnostics: System Activity` & `Status: Traffic Graph` only.  Finally, create a new API key which will download an `apikey.txt` file with your key and secret in it.  Paste the contents following the = symbol in both lines to the username and password fields below.
 
-Allowed fields: `["uptime", "cpu", "memory", "wanUpload", "wanDownload"]`.
+Allowed fields: `["cpu", "memory", "wanUpload", "wanDownload"]`.
 
 ```yaml
 widget:
     type: opnsense
     url: http://opnsense.host.or.ip
-    username: apikey
-    password: apisecret
+    username: key
+    password: secret
 ```
 
 *Added in v0.5.6*