A line in layout.htmlpage.menu.php:

(new CLink(SPACE, 'http://www.zabbix.com/documentation/3.0/'))

is potentially may be forgotten to be updated for next major releases.

I believe it should be generated automatically, taking version form defines.inc.php
ZABBIX_EXPORT_VERSION could be used, but most likely a new define is required.

Maybe just would worth to move the link itself to defines.inc.php, near to other defines related to versions. This way it will be noticed every time when preparing a new release, I hope.