Make element templates an all modelers feature

[nikku]

Describe the context

Element templates are nowadays an all modelers BPMN modeling feature:

• They are supported in the desktop modeler with discovery through local file system discovery
• They are supported in the web modeler (branded as "connector templates") as first class citizens

Still, to date they are burried under "Modeler > Desktop Modeler" with a note hinting on desktop modeler support (https://docs.camunda.io/docs/components/modeler/desktop-modeler/element-templates/about-templates/).

Various deep links from different places in the documentation link to sub-pages, i.e. Web Modeler > Advanced Modeling > Managing Connector Templates. These often don't establish the necessary context, i.e. they are referred to as "connector templates" from where they are linked; we don't introduce element templates = connector templates properly (i.e. here).

Describe the solution you'd like

• Move element templates one level up (under Modeler)
• Clarify prominently that element templates = connector templates ("Element templates, also referred to as connector templates in some parts of our product")
• Add dedicated section on how to use (in both modelers; link to respective modeler sections (https://docs.camunda.io/docs/next/components/modeler/web-modeler/advanced-modeling/manage-connector-templates/, ...))
• Establish link to connectors

Which alternatives did you consider?

Keep things as is.

Further context

Confusion around this came up repeatedly. Cf. example.

[nikku]

@YanaSegal Please review, I think it should match what we discussed.