fix: remove broken link to non-existent FES performance test

[Jah-yee]

Description

Remove the broken link to the "Friendly Error performance test" in the FES contribution guide. The linked path (src/assets/learn/performance/code/friendly-error-system) no longer exists and returns 404.

Fix

The link in src/content/contributor-docs/en/fes_contribution_guide.mdx pointed to a non-existent directory. Since the referenced content has been removed or moved, the broken link is removed while preserving the documentation of FES performance impact.

Fixes #1578

[Nwakaego-Ego]

Hi @Jah-yee, thank YOU for the work on this! In the future, we ask that you wait to be assigned before working on any issues. community guidelines for more information. In this case, the issue was still being discussed, and its author had already volunteered to work on it. Following the assignment process helps us avoid duplicated work and ensures that contributors who have already volunteered are not passed over.

This PR changes src/content/contributor-docs/en/fes_contribution_guide.mdx, which is created from the contributor documentation in the processing/p5.js repository. The next documentation sync could replace this change and bring back the broken link. The lasting fix needs to be made in the upstream contributor_docs/fes_contribution_guide.md file.
Because this PR was opened before the issue was approved and assigned, and it changes the generated copy instead of the original document, we will close it and manage the fix through issue #1578.