Update the Product Designer Onboarding template

While working on a new team member's general onboarding and onboarding plan, I noticed that there are a number of tasks replicated across both the company onboarding and product designer onboarding issues.

I also took into account the feedback from Bonnie's recent onboarding experience.

This MR attempts to remove any inconsistencies to the templates, but also:

• Introduces a "Content" section where readers can see at a glance what the issue entails
• Groups the content of the onboarding by section, to help readers navigate the issue
  1. UX Department
  2. Your product stage
  3. Product Designer workflows
  4. Product, technology and personas
  5. Tips and tricks
  6. Feedback on your onboarding experience
• Makes some of the language less harsh and highlights items that are optional. For example, the current description tells the new team member to read the whole ux handbook. That task can create anxiety and is not the most useful way to use someone's time. It’s impossible to know everything.
• Includes a subsection for collaborating with TW
• Includes the google calendar IDs to the description.
• Removes outdated links
• Standardises "UX Buddy" mentions across the text
• Hide the tips and feedback checkboxes inside an expandable component, to remove noise from the text