Add Xquik and repair the contribution guide

[kriptoburak]

Summary

• Add Xquik to Social Networks for public X data OSINT workflows.
• Add the required independent-service and trademark disclosure.
• Replace the broken contribution-guide link with a real CONTRIBUTING.md.
• Document duplicate checks, entry format, official-link preference, validation, and CC0 terms.
• Repair the broken Back to top link with a stable explicit anchor.
• Remove the stray empty README heading.
• Rebase the complete change into 1 current, SSH-signed commit.

Validation

• markdown-link-check README.md --quiet
• markdown-link-check CONTRIBUTING.md --quiet
• git diff --check
• Edited-file secret and forbidden-punctuation scans
• awesome-lint: 8 findings on this branch and 8 on current upstream, so this change adds no finding
• Xquik repository and license links return HTTP 200

Xquik is an independent third-party service. Not affiliated with X Corp. "Twitter" and "X" are trademarks of X Corp.

[kriptoburak]

Repair published in signed commit bdddbc3.

The branch now has exactly 1 current commit. It preserves the Xquik OSINT entry and independently repairs the missing contribution guide, broken contribution link, broken Back to top link, and stray empty heading.

Validation passed:

• Markdown links in README.md and CONTRIBUTING.md
• git diff integrity
• edited-file credential and punctuation checks
• awesome-lint parity: 8 branch findings and 8 current-upstream findings

The published commit has a verified GitHub signature.