DRAFT: Make first-time contribution doc easier for first-time contributors

What does this MR do and why?

Edit and reorganize Tutorial: Make a GitLab contribution to make it easier for first-time contributors.

This is the first among a series of edits I have planned or in process.

Changes in this MR:

• test and copy edit first_contribution
  • new heading to describe the two options
  • separated the Gitpod and GDK steps into distinct H3
  • more strongly highlight Gitpod + Community fork
• repeatable steps for GDK

This is the first in a multi-stage process. Future MRs:

• suggest quick changes use Community Fork and Web IDE: gitlab-community/gitlab!16 (closed)
• GDK-in-a-box
• GDK troubleshooting needs to go somewhere else, even if it's at the end of the page
  • marking as resolved in gitlab-community/gitlab!16 (closed) for now, but I think the greater issue here is that the GDK docs are spread around and could use some consolidation.
• re-organize into multi-page guide: gitlab-community/gitlab!16 (closed)
• Contributor landing page?
  • "I want to":
    • correct something in the code (quick change)
    • add a feature (Gitpod or GDK)
    • file an issue
    • assign myself a specific MR
    • find an MR to help with

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