[Doc] Update 5.0 to 5.1: Recompile assets

Recompiling assets on the update is needed or main menu appearance may not load properly. This has been mentioned on issues #3722 and #3978.

The command has been tested several by users on those issues and by myself.

Thanks for your time and attention.
Merge pull request #4369 from zeehio/patch-1

[Doc] Update 5.0 to 5.1: Recompile assets
Merge branch 'master' of github.com:jackbot/gitlabhq