Troubleshooting: can't log in (#17558)

* Outline for login troubleshooting guide

* Update docs/troubleshooting-guide/cant-log-in.md

Co-authored-by: Jeff Bruemmer <jeff@metabase.com>

* Update docs/troubleshooting-guide/cant-log-in.md

Co-authored-by: Jeff Bruemmer <jeff@metabase.com>

* Importing material

* Prosifying

* Updates based on feedback

* Another update to login troubleshooting

* Fixing typo

* One more update

Co-authored-by: Jeff Bruemmer <jeff@metabase.com>