Docs: Fix broken links from steward guidelines page

[lucyhartigan]

Addresses Issue #527

Links from the steward guidelines to the design principle and contributor guidelines documentation redirect to a 404 page rather than the intended content.

Changes

• update/fix multiple broken links from steward guidelines to p5 design principles
• update/fix broken link from steward guidelines to p5 contributor guidelines

PR Checklist

• npm run test passes

[limzykenneth]

The content for the contributor docs is built off of the source in the main p5.js repo. Changing the link here will cause the link to then be overwritten but the version in the p5.js repo and thus reverting it. Changing the link in the p5.js repo is also not the solution as it will break the link there in the source.