fix: Stealth forwarders concept + tutorial

[BIGSMKE12]

Summary

This pull request implements the changes requested in #130.

Changes

• guides/stellar/stealth-forwarders.mdx

Testing

Verified against the issue acceptance criteria.

Closes #130

[truthixify]

Thanks @BIGSMKE12. Right now the whole PR is one line:

# Stealth Forwarders

No frontmatter, no body, which is also why Compile docs snippets is failing.

Issue #130 asks for the concept plus the tutorial: why forward and when it actively hurts privacy, a reference implementation walkthrough for the watcher and relay, fee accounting for who pays the forwarding hop, and the trust boundary covering what the forwarder learns.

No rush on the timeline, but please push the actual content before this is ready to look at. If you have hit something blocking, say so here and I will help. If you would rather hand the issue back, that is also completely fine, just let me know so it can go to someone else.

When you do push, start the file with frontmatter matching the neighbouring guides in guides/stellar/, which is what the snippets job expects.