docs: add contributing guide and fix stale contribution links

[ming1523]

Summary

This PR fixes the repository's contribution entrypoints and removes a few stale contribution references.

Changes

• add a root CONTRIBUTING.md as the canonical contribution guide
• fix broken contribution links in the relayer docs
• update the coprocessor contribution page to point to the current zama-ai/fhevm issue tracker
• fix a small typo in the CLA sentence

Why

At the moment:

• the relayer docs point to a non-existent root CONTRIBUTING.md
• .github/CONTRIBUTING.md is about GitHub workflows, not contributor onboarding
• the coprocessor contribution page still references the old fhevm-backend issue tracker

This makes the contribution path harder to discover for new external contributors.

Validation

• verified the updated contribution links point to existing files
• verified the stale fhevm-backend issue link is removed from the touched docs

[cla-bot]

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[ming1523]

I have signed the individual CLA. Please let me know if anything else is needed from my side.

[ming1523]

Follow-up on the CLA check: I signed the individual CLA on April 1, 2026, and I have now added the same note across the related PRs in this small series (#2226, #2227, #2228, #2230, #2231, #2232, #2233). If the CLA has already been received on your side, could someone please refresh or re-run the CLA status for these PRs? Thank you.

[github-actions]

This PR is unverified and has been open for 2 days, it will now be closed. If you want to contribute please sign the CLA as indicated by the bot.

[aquint-zama]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[aquint-zama]

Follow-up on the CLA check: I signed the individual CLA on April 1, 2026, and I have now added the same note across the related PRs in this small series (#2226, #2227, #2228, #2230, #2231, #2232, #2233). If the CLA has already been received on your side, could someone please refresh or re-run the CLA status for these PRs? Thank you.

everything ok @ming1523 sorry for the delay