Document Kestrel breaking changes, new API and new config in 5.0

[halter73]

We need to update/write conceptual docs to reflect breaking changes, new API and new config in 5.0.

1. New kestrel event counters (Added some kestrel event counters #21649)
   a) Related: Add docs on the EventCounters emitted by ASP.NET Core AspNetCore.Docs#18272
2. PEM support ([HTTPS] Adds PEM support for Kestrel #23584)
   a) Related: Add section for loading PEM keys AspNetCore.Docs#19113
3. Automatic config reload (Kestrel reloadable endpoint config #21072 & Enable Kestrel config reload in ConfigureWebHostDefaults #22528)
4. SNI from config (Kestrel SNI from config #24286)
5. Async ServerOptionsSelectionCallback UseHttps() overload (Add async ServerOptionsSelectionCallback UseHttps overload #25390)
6. Use platform-default TLS/SSL protocol versions by default (Use default SslProtocols in Kestrel #22437)
7. Obsolete the Libuv transport (Obsolete the Libuv transport #23480)
8. "SslProtocols" settable via config (Kestrel Endpoints' "SslProtocols" settable via config (#22663) #22910)
9. "ClientCertificateMode" settable via config (Load ClientCertificateMode from config #24076)

The following APIs are new, but might not warrant conceptual docs.

1. custom request header decoder API to Kestrel (Add custom request header decoder API to Kestrel #23233)
2. Inline scheduler option for Sockets transport (Add inline scheduler option for Sockets transport #24638)
3. Add SocketTransportOption to enable/disable WaitForData (Add SocketTransportOption to enable/disable WaitForData #19396)

@Pilchie Would it be better to move this to https://github.com/dotnet/AspNetCore.Docs?

[Pilchie]

Tagging @mkArtakMSFT for thoughts on whether this should move to the Docs repo or stay here. Personally I spend a lot more time here.

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[davidfowl]

@halter73 seems like we didn't fix the KestrelServer constructor breaking change. Is that being documented in here as well?

[halter73]

We should fix it.