Update Active Directory Docs

[domenicsim1]

• Removes the incorrect statement that Kestrel is required for High Availability configurations,
  HTTP.sys works fine for HA. Kestrel is only required if you want to use Kerberos authentication.
• Updates the supported auth scheme table to reflect what actually works with each web server mode
  (HTTP.sys vs Kestrel), rather than single server vs HA.
• Removes the statement that Kerberos is unsupported in HA, it works in HA when using Kestrel.
• Adds two new troubleshooting entries based on a recent customer case:
  • HTTP.sys masking errors and returning a generic 500 (with Kestrel as a diagnostic workaround)
  • Maximum Session Duration set too low silently breaking AD SSO

[domenicsim1]

The files have not been changes in a longtime so many pre-existing issues got surfaced hence why many changes.

[colinbowern]

LGTM