Update Bitgesell contributing guide

[JeremyZeng77]

Problem

CONTRIBUTING.md still describes libsecp256k1 rather than Bitgesell Core. This makes the contributor guide misleading for new contributors and leaves several local markdown references pointing at files that do not exist in this repository.

Summary

• Retarget the guide title, scope, contribution expectations, and communication section to Bitgesell Core.
• Replace secp256k1-specific C89/API/test guidance with Bitgesell Core-oriented C++20, consensus/wallet safety, and test guidance.
• Update peer-review and consensus wording from Bitcoin Core/IRC-specific language to Bitgesell Core/GitHub/community wording.
• Replace the external Bitcoin translation-process link with the local Bitgesell translation guide.

Verification

• git diff --check
• Checked for stale libsecp256k1, secp256k1, C89, IRC, and Bitcoin consensus wording in CONTRIBUTING.md.
• Checked local markdown links in CONTRIBUTING.md; all local links resolve.

Related bounty: #32 and #81

Payment if approved: USDT TRC20 TWupTAoFjxR2VmqUUvzhfmUwXVZR6Kvz1Y.