Sttp documentation audit

[Pask423]

It seems that sttp documentation contains some broken links. It will be good to take a closer look at existing documentation under https://sttp.softwaremill.com/en/latest/ and verify if everything works correctly. Maybe some parts of documentation are not needed anymore. Wait until https://github.com/softwaremill/sttp/pull/1409 is merge as it contain lots of changes in documentation which also should be checked