Update element template docs

[pinussilvestrus]

What should we do?

On the same run as adding the C8 element templates support, we have to make sure that we update our docs. That includes

• Update screenshots
• Add Cloud samples
• Document the usage of the $schema attribute, mark Cloud templates
• Add new / update features
  • groups
  • optional properties (only support zeebe:input and zeebe:output
  • "new" entries visible = true
  • Cloud bindings
  • zeebe:taskDefinition
  • zeebe:taskHeader
  • zeebe:input
  • zeebe:output
• Split up binding docs between Platform and Cloud (cf. Update element templates docs to indicate Platform #2382)
• Mark error scopes as Platform only

Why should we do it?

Keep things up to date
Child of #2640

[pinussilvestrus]

Updates after chatting with @MaxTru (24th Feb)

https://miro.com/app/board/uXjVOKUvt2s=/

General

• Most parts of our docs don't need updates, as the element templates feature works the same in both worlds
• We consider moving our docs to https://docs.camunda.io/docs/components/modeler/camunda-modeler/element-templates/, but this is a general decision to be made
• We don't use our screenshot tool, as we believe the screenshots will keep static
• We choose Platform and Cloud as naming, to be renamed in April release (general topic)
• We will keep our example snippets Platform specific, adding Cloud snippets when needed

Things to be updated / added

• Using templates
  • Update template properties to explain both Platform and Cloud
• Defining template
  • $schema: explain how to mark Cloud template
  • add grouping as feature
  • add optional properties as feature
  • mark error scopes as Platform only
  • add "new" entries visible = true feature
  • add zeebe bindings, split up bindings section in general