Add EnvelopeStatus extensions and update documentation URL

The `System` namespace was added to `EnvelopeStatus.cs` to enable additional functionality. A documentation URL in the comments was updated to point to a new location, replacing the outdated link.

Introduced a new static class `EnvelopeStatusExtensions` with two extension methods for the `EnvelopeStatus` enum:
- `IsActive`: Checks if the status is active (between `EnvelopeCreated` and `EnvelopePartlySigned`).
- `IsCompleted`: Checks if the status is completed (between `EnvelopeCompletelySigned` and `EnvelopeWithdrawn`).

EnvelopeGenerator.API/Properties/launchSettings.json

@@ -22,7 +22,7 @@
     "https": {
       "commandName": "Project",
       "dotnetRunMessages": true,
-      "launchBrowser": false,
+      "launchBrowser": true,
       "launchUrl": "swagger",
       "applicationUrl": "https://localhost:8088;http://localhost:5131",
       "environmentVariables": {