Refactor "Install FlowFuse on Kubernetes" documentation

[ppawlowski]

From notes:

• documentation is chaotic
  • some information about possible configuration is showed after installation is made (enabling mqtt and file-store), this is confusing
  • some very specific configurations examples are mentioned which may increase the confusion
• Configuring Flowfuse/First Run Setup - we should consider moving it to the same level as Installing Flowfuse and Upgrading Flowfuse and use it as a starting point for configuration-related FAQ
• typo in forge.localPostrgresql
• This is done by creating a customization.yml file in the helm directory - there is no need to create helm directory and keep customization.yml there.

Like with Docker setup, I would aim to make installation as simple as possible. I would hide stuff related to more specific configuration behind drop-down FAQ or dedicated configuration-related page.