Update Documentation for 7.6 release

Updated the update and install documentation for the 7.6 release.

See merge request !1334

Credentials in readme

See merge request !1332

Dev docs in gdk

Also see https://gitlab.com/gitlab-org/gitlab-development-kit/merge_requests/31

See merge request !1331

update order of upgrade guide

Add kerberos to gemfile

See merge request !1330

Add documentation for the Sidekiq MemoryKiller

We already merged the code, and we are already using it on gitlab.com. This MR adds documentation.

See merge request !1325

Update changelog 7.6

See merge request !1327

Kerberos dependency

See merge request !1324

Fix note update

In some strange cases Ruby thinks `system` is not AR field but
Kernel.system call and raises exception during save. It should prevent
this random wierd behaviour.

Exception log:

```
ArgumentError (wrong number of arguments (0 for 1+)):
  app/models/note.rb:505:in `system'
  app/models/note.rb:505:in `editable?'
  app/controllers/projects/notes_controller.rb:33:in `update'
```

See merge request !1326

In some strange cases Ruby thinks `system` is not AR field but
Kernel.system call and raises exception during save. It should prevent
this random wierd behaviour
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Added process for green tests

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>

Fixed a lot of already defined notices

Added Code of Conduct

Add missing webhook doc for tag event

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>

remove extra upgrade details, add missing config

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>

Fix typo in the README.md for docker

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>

add details on backing up your SSH host keys

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>