doc: update secp256k1 documentation links

[zhanzhao-h]

Description

Updates stale secp256k1 documentation links.

The project already vendors secp256k1 under src/secp256k1; this points contributors and readers to the in-tree documentation instead of the old external Bitcoin Core path.

Notes

Docs-only change.

BTC/BGL PR reward address

To be provided if applicable.

Validation:

• git diff --check origin/master..doc/fix-secp256k1-doc-links
• rg -n "bitcoin-core/secp256k1|bitcoin-core/secp256k1/blob/master" README.md CONTRIBUTING.md

Refs #32

[MyTH-zyxeon]

Maintainer review note for the #32/#81 bounty queue:

This docs-only PR looks safe and useful: I checked the current head db67f536 and each replacement target exists in-tree:

• src/secp256k1/include/secp256k1.h
• src/secp256k1/src/tests_exhaustive.c
• src/secp256k1/examples/ecdsa.c
• src/secp256k1/examples/schnorr.c
• src/secp256k1/examples/ecdh.c

The acceptance boundary I would use is narrow: keep this as link-retargeting only, with no copied upstream secp256k1 content and no semantic build/test guidance changes beyond pointing readers to the vendored Bitgesell tree. The PR body already has git diff --check plus stale-link grep coverage; a quick rendered Markdown check on README/CONTRIBUTING should be enough for final docs review.

No wallet, node, key, dependency, or live-chain scope is involved.

[MikeGarciaAGM]

Looks good. The docs links now point to in-tree Bitgesell paths and the change is clean.