docs: add build step to contributing setup instructions

[carlesandres]

Description

Adds the missing pnpm build step to the setup instructions in CONTRIBUTING.md. Without this step, new contributors will encounter test failures due to unbuilt internal packages (e.g., remend).

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Refactoring (no functional changes)

Related Issues

N/A - Discovered while following the setup instructions as a new contributor.

Changes Made

• Added step 4: pnpm build (required before running tests)
• Renumbered the test step to step 5

Testing

• All existing tests pass
• Added new tests for the changes
• Manually tested the changes

Verified that following the updated instructions allows tests to pass successfully.

Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• I have created a changeset (pnpm changeset)

Changeset

• N/A - Documentation-only change, no package version bump needed

Additional Notes

The error that occurs without this step:

Error: Failed to resolve entry for package "remend". The package may have incorrect main/module/exports specified in its package.json.

This happens because internal packages like remend need to be built before the streamdown package tests can resolve them.

[vercel]

@carlesandres is attempting to deploy a commit to the Vercel Team on Vercel.

A member of the Team first needs to authorize it.

[haydenbleasel]

Nice one, appreciate it!