docs: Fix link to style guide in contributing.md

[Raniz85]

It is now absolute, so works both when viewed on the repository index page (i.e. the Contributing tab) and when navigating to docs/contributing.md directly.

Checklist

If applicable:

• I have updated CHANGELOG.md (This is a documentation typo, so I don't think it warrants a changelog entry)
• I have updated the documentation (README.md, docs/, demos/) 🙃
• I have updated the config schema (cli/src/config-schema.json) - N/A
• I have added/updated tests to cover my changes - N/A
• I fully understand the code that I am submitting (what it does,
  how it works, how it's organized), including any code drafted by an LLM.
• For any prose generated by an LLM, I have proof-read and copy-edited with
  an eye towards deleting anything that is irrelevant, clarifying anything
  that is confusing, and adding details that are relevant. This includes,
  for example, commit descriptions, PR descriptions, and code comments. No LLMs involved here

[PhilipMetzger]

This still has the same problem as #8561 which was someone else attempt.