docs(dotnet): add Web.config DSN example for ASP.NET

[fuguiKz]

Resolves getsentry/sentry-dotnet#4777

Adds a basic Web.config appSettings snippet and updates the ASP.NET guide example to read the DSN from ConfigurationManager.AppSettings["SentryDsn"].

Urgency: not urgent.

[vercel]

@fuguiKz is attempting to deploy a commit to the Sentry Team on Vercel.

A member of the Team first needs to authorize it.

[sentry]

Bug: The documentation's code example for ASP.NET will crash the application on startup if the SentryDsn key is missing from Web.config due to an unhandled ArgumentNullException.
_{Severity: CRITICAL | Confidence: High}

🔍 Detailed Analysis

The code example in the documentation for ASP.NET initialization uses options.Dsn = ConfigurationManager.AppSettings["SentryDsn"];. If the SentryDsn key is missing from Web.config, ConfigurationManager.AppSettings returns null. In Sentry SDK 4.0+, passing a null DSN to SentrySdk.Init() throws an ArgumentNullException. Because this initialization code is placed in Application_Start(), the unhandled exception will prevent the application from starting. The documentation fails to warn users that this configuration key is mandatory, making it easy for a developer following the guide to cause a startup crash.

💡 Suggested Fix

Update the documentation to include a prominent warning that the SentryDsn key in Web.config is mandatory. Alternatively, modify the code example to include null-checking before assigning the DSN to options.Dsn to prevent the SentrySdk.Init() call from throwing an exception if the key is not found.

🤖 Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: docs/platforms/dotnet/guides/aspnet/index.mdx#L64

Potential issue: The code example in the documentation for ASP.NET initialization uses
`options.Dsn = ConfigurationManager.AppSettings["SentryDsn"];`. If the `SentryDsn` key
is missing from `Web.config`, `ConfigurationManager.AppSettings` returns `null`. In
Sentry SDK 4.0+, passing a `null` DSN to `SentrySdk.Init()` throws an
`ArgumentNullException`. Because this initialization code is placed in
`Application_Start()`, the unhandled exception will prevent the application from
starting. The documentation fails to warn users that this configuration key is
mandatory, making it easy for a developer following the guide to cause a startup crash.

_{Did we get this right? 👍 / 👎 to inform future reviews.
Reference ID: 7752030}