documentation

Merge branch 'release/6.0' of https://git.zabbix.com/scm/web/documentation into feature/DEV-3890-6.0-D. Merge conflict resolved in:
- /en/manual/api/reference/alert/get.md (updated blockID);
- /en/manual/api/reference/mediatype/get.md for countOutput (caused by a6c44ad24da).