Add Passkeys Guide (#455)

* add passkey post

* remove useless anchor

* remove blank line

* Update server/guides/webauthn.md

Co-authored-by: Tim Condon <0xTim@users.noreply.github.com>

* Update server/guides/webauthn.md

Co-authored-by: Tim Condon <0xTim@users.noreply.github.com>

* small adjustments

* rename webauthn file to passkeys

* add passkey entry to guides index

* minor fixes

* change Swift to swift

* minor fixes

* Update server/guides/passkeys.md

Co-authored-by: Alexander Sandberg <hi@alexandersandberg.com>

* Update server/guides/passkeys.md

Co-authored-by: Alexander Sandberg <hi@alexandersandberg.com>

---------

Co-authored-by: Tim Condon <0xTim@users.noreply.github.com>
Co-authored-by: Alexander Sandberg <hi@alexandersandberg.com>

Author: 3 people

assets/images/server-guides/1password_passkey_prompt.png

@@ -16,6 +16,7 @@ They focus on how to compile, test, deploy and debug such application and provid
 - [Debugging multithreading issues and memory checks](/documentation/server/guides/llvm-sanitizers.html)
 - [Deployment](/documentation/server/guides/deployment.html)
 - [Packaging](/documentation/server/guides/packaging.html)
+- [Passkeys](/documentation/server/guides/passkeys.html)
 
 Additionally, there are specific guides for library developers: