Fix broken SDK docs link

[Hopium21]

Replaced outdated link https://sdk-docs.nexus.xyz which leads to a blank placeholder page with a more specific and functional one:
🔗 https://sdk-docs.nexus.xyz/doc/nexus_sdk/index.html

📌 Reason:
The previous link only showed a placeholder ("Go to SDK docs") and did not provide actual documentation. The updated link leads directly to the SDK reference and guides.

📷 Screenshot (for reference):

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[Hopium21]

I have read the CLA Document and I hereby sign the CLA

[sjudson]

@Hopium21 your signature seems to have not taken, maybe try again?

[Hopium21]

I have read the CLA Document and I hereby sign the CLA

[Hopium21]

@Hopium21 your signature seems to have not taken, maybe try again?

Successful