Docs: update handbook dev guidelines assignment

Why is this change being made?

For gitlab-org/technical-writing#108 (closed):

• Explain what dev guidelines must be reviewed by a TW.
• Minor updates:
  • Update list of TWs
  • Explain that the TW assigned to a stage owns the docs for that stage
  • Reorg headings for better doc outlines

References

• All Dev Guidelines: gitlab-org/technical-writing#108 (closed)
• Pajamas: gitlab-org/technical-writing#93 (closed), gitlab-org/gitlab-services/design.gitlab.com#472 (closed)
• GitLab UI: gitlab-org/gitlab-ui#598 (closed), gitlab-org/gitlab-ui#624 (closed)

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing? => Mike Lewis

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what