docs: add non-blocking platform PR process guide

[georgewrmarshall]

Description

Adds docs/non-blocking-platform-pr-process.md, a process guide for reviewing consumer-repo PRs (e.g. extension, mobile) when they introduce UI that touches design-system-shaped components but MMDS has not yet defined the variant, token, or API (e.g. first warning styling on a TextField).

The doc covers:

• Why: Avoid making the design system a merge gate while still capturing custom overrides and gaps for later systemization.
• How: Do not block merge; suggest a localized approach (e.g. Tailwind border-warning-default rather than new SCSS modifiers where appropriate); add a grep-friendly TODO tagging @MetaMask/design-system-engineers; open a Slack thread for async DS discussion; follow up in MMDS and consumer repos once canonical behavior exists.
• Reference: CONF-1099 and metamask-extension#41196, including the inline review on recipient-input.tsx.

Related issues

Fixes: (none — documentation only)

Manual testing steps

1. Open docs/non-blocking-platform-pr-process.md in the repo or on GitHub and confirm headings, links, and tables render as expected.
2. (Optional) Run yarn prettier --check docs/non-blocking-platform-pr-process.md.

Screenshots/Recordings

Documentation-only change; no UI.

Before

N/A

After

N/A

Pre-merge author checklist

• I've followed MetaMask Contributor Docs
• I've completed the PR template to the best of my ability
• I’ve included tests if applicable (N/A — markdown only)
• I’ve documented my code using JSDoc format if applicable (N/A)
• I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

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Note

Low Risk
Documentation-only change that adds a process guide; no runtime behavior, APIs, or data flows are modified.

Overview
Adds a new doc, docs/non-blocking-platform-pr-process.md, defining a non-blocking review process for extension/mobile PRs that need UI beyond current MMDS support.

The guide standardizes using localized overrides plus a grep-friendly // TODO: @MetaMask/design-system-engineers: marker and a Slack handoff to track gaps and follow up asynchronously after merge.

^{Written by Cursor Bugbot for commit acae346. This will update automatically on new commits. Configure here.}

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