Add preliminary 7.6 upgrade guide. Makes it easier to add upgrades as changes are made rather than trying to round up everything at RC1.

Initial additions:
* Nginx changes needed again in 7.6 as they did not make the final 7.5 upgrade guide
* Suggest that user sets time zone (added in https://github.com/gitlabhq/gitlabhq/pull/8015 but missed in final 7.5 upgrade guide)

Replaces https://github.com/gitlabhq/gitlabhq/pull/8124