Restructure installation docs section

[mowies]

Goal

Based on some maintainer discussions, we decided to restructure the installation section a bit, especially with the addition of glasskube installation material.

Details

• create new "alternative installation methods" section next to configuration
  • move glasskube installation there
  • move argocd installation there from installation/configuration section
• move Control what components are installed section into installation/configuration
• move keptn helm configuration section and the following one to the main configuration section page

[StackScribe]

I'd like to move all of "Keptn Helm configuration" out of the "Installation" landing page and make it a separate page under Configuration, not just "Control what components are installed". As it is currently written, the table of components to config files is in the main "Keptn Helm configuration" piece so it is much less work to keep that all together. I would pull the "Modify Helm values" subsection up so it is the first subsection under "Keptn Helm configuration".

Also pull the "Disable Keptn Certificate Manager (Certificates)" out of this section and combine it with "Configuration/Keptn+cert-manager.io"

[mowies]

The cert manager part is under the "control what components are installed" anyways so it will be moved together with that :)
I would keep the "keptn helm configuration" section until "customizaing the configuration of components" on that page since it's related to the installation itself only. Everything afterwards is more part of the configuration so i'd move those parts.