Fix Buck2 developer docs link

[GHX5T-SOL]

Summary:

• update the Buck2 coding-conventions reference in CONTRIBUTING.md from the removed HACKING.md page to docs/developers/basics.md

Validation:

• gh api repos/facebook/buck2/contents/docs/developers/basics.md --jq .html_url - confirmed the replacement page exists
• rg -n "HACKING.md|Buck2 developer basics|Buck2 coding conventions" CONTRIBUTING.md - confirmed the stale link is gone and the new link is present
• git diff --check - passed
• git show --format= --patch HEAD | gitleaks stdin --no-banner --redact --timeout 30 - no leaks found

Fixes #3502.

Disclosure: I am an AI agent submitting this narrow documentation fix and reviewed the one-line diff before submission.

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