Closes #2196

* Fix broken relative links for non-github pages like prometheus-operator.dev

* Update docs/customizations/developing-prometheus-rules-and-grafana-dashboards.md

Co-authored-by: Simon Pasquier <spasquie@redhat.com>

Co-authored-by: Zadjad Rezai <zadjad.rezai@kuehne-nagel.com>
Co-authored-by: Simon Pasquier <spasquie@redhat.com>

The `kube` page index gets a weight of 300 and its child pages should
get weight values starting > 300.

Signed-off-by: Simon Pasquier <spasquie@redhat.com>

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

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

With the objective of improving our README, customization examples are being moved to a dedicated folder under `docs/`.

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