Add Link response headers for agent discovery

[kaankacar]

Add Link HTTP headers to developers.stellar.org for agent discovery

Problem

No Link headers are returned on the homepage today:

curl -sSI https://developers.stellar.org/

shows none, so agents can't discover the site's machine-readable resources via standard HTTP link relations.

Fix

In nginx/nginx.conf (the prod server, per the Dockerfile's nginx:1.29 stage), add an add_header Link "..." always; directive inside location / that advertises:

Resource	Relation
/stellar-rpc.openrpc.json	service-desc
/docs/data/apis/horizon	service-doc
/docs/data/apis/anchor-platform-api	service-doc
/docs/data/apis/stellar-disbursement-platform-api	service-doc
/llms.txt	describedby
/.well-known/api-catalog (future)	api-catalog (per RFC 9727)