Update and finalize UXW style guide

[seanlip]

Platform
Design style guide

Describe the request

We have a style guide for UXW at https://docs.google.com/document/d/1poCxa3ys_X6ef2GsJHS_mDbC-MOkdZoCccHDw-xLQ4g/edit?tab=t.0#heading=h.ehrzz7fue6cs, linked from the README at https://github.com/oppia/design-team?tab=readme-ov-file#design-guides.

The aim of this issue is to audit that style guide and make sure it is up to date, so that UXW and other teams can use it as a basis for future writing projects.

See UXW Style Guide Revamp Planning Doc here: https://www.notion.so/1071fe247c1e80da84d0d6e333635cb9?v=45658707ddc949bc8989e9577f4c716a&p=1171fe247c1e804fa7bde1e10c2ab00e&pm=c

UXR success criteria

• UXW team members can rely on the style guide when doing UXW projects
• Developer and design team members can refer to the style guide to get an understanding of Oppia's writing requirements.

DESIGN APPROVAL CHECKLIST -- DO NOT EDIT.

• UX Design approved (Android mocks should include dark mode)
• UX Writing approved
• Product approved

For a design project to be completed, it needs approval from UXD, UXW and PM. Please tag the following reviewers when your project is ready (at least one reviewer for each of the bullet points below):

• UXD: @rflore / @S4v8n (for Web projects), @seanlip (for Android projects)
• UXW: @tamiorendain
• PM: @seanlip