Update Handbook entries on Data Platform: ZenDesk pipelines change to Meltano

What does this MR do?

This MR updates the implementation details for ZenDesk and ZenDesk Community Relations, after they have been moved from Stitch to Meltano, as per:

• #9579 - Change Zendesk extraction to Meltano
• #11678 - Change Zendesk Community Relations extraction from Stitch to Meltano

Additionally, it adds a section for each of them at the end of the Data Platform / Pipelines handbook page with implementation details.

Related issues

• Closes #11929