docs: fix dead links in contributing guide

[sugarhush]

Description

Replace inaccessible external links in the contributing documentation.

Changes

• Replace the dead LUFA website link with the official LUFA GitHub repository.
• Remove the inaccessible "How To Ask Questions The Smart Way" (catb.org) link.
• Update the wording from "these are some good guides" to "this is a good guide".

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout (addition or update)
• Documentation

Issues Fixed or Closed by This PR

• Fixes Inaccessible external links in "Contributing" documentation #26368

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).