*: add test of mixin addition in examples/; change config to `_config` in addMixin to be consistent with main components

Signed-off-by: Damien Grisonnet <dgrisonn@redhat.com>

Replace Watchdog alerts part of the `example-group` in some examples by
ExampleAlert alerts to reinforce the fact that this is just an example.

Signed-off-by: Damien Grisonnet <dgrisonn@redhat.com>

Signed-off-by: ArthurSens <arthursens2005@gmail.com>

Signed-off-by: Damien Grisonnet <dgrisonn@redhat.com>

Signed-off-by: Damien Grisonnet <dgrisonn@redhat.com>

Signed-off-by: James Sturtevant <jstur@microsoft.com>

Signed-off-by: paulfantom <pawel@krupa.net.pl>

Signed-off-by: paulfantom <pawel@krupa.net.pl>

Signed-off-by: paulfantom <pawel@krupa.net.pl>

Signed-off-by: paulfantom <pawel@krupa.net.pl>

Signed-off-by: paulfantom <pawel@krupa.net.pl>

Signed-off-by: Kemal Akkoyun <kakkoyun@gmail.com>

Signed-off-by: Kemal Akkoyun <kakkoyun@gmail.com>

Signed-off-by: Kemal Akkoyun <kakkoyun@gmail.com>

Signed-off-by: Kemal Akkoyun <kakkoyun@gmail.com>

Removed EKS patch from example

This could have been achieved either by switching to stringData, or doing
`std.base64(std.encodeUTF8($._config.alertmanager.config))` as per
google/jsonnet#575

I went with the former, because it's:
1. Easier to read existing config
2. Consistent with the way jsonnet object-based config is written just above

Instructions to add Grafana dashboard do not work. The proposed
functions are wrong, according to
[grafana.libsonnet](https://github.com/brancz/kubernetes-grafana/blob/master/grafana/grafana.libsonnet)
`dashboards` and `rawDashboards` should be used in `grafana+::`
field.

This PR updates the existing documentation and fixes minor typos.

Fixes #455.

https://github.com/coreos/kube-prometheus/pull/425#pullrequestreview-368779890

Review comment: https://github.com/coreos/kube-prometheus/pull/425#discussion_r387494885