docs: add Wave registry access troubleshooting page

[MichaelTansiniSeqera]

Summary

Adds docs/troubleshooting.md — a new page covering the most common Wave registry access failure.

What's documented

Container image does not exist or access is not authorized

The Platform credential-validation error Invalid credentials for container registry is misleading: it fires on any Wave-to-registry failure including network unreachability, not only bad credentials. The Wave API 400 response has the same ambiguity. This page explains:

• What both error surfaces look like and why the wording is misleading
• How to distinguish a credential failure from a network reachability failure
• That hosted Wave runs in Seqera's cloud and cannot reach private/internal registries regardless of credentials
• That self-hosted Wave is the standard solution for internal registries, with a link to the Kubernetes installation guide

Related

• Sidebar entry added in seqeralabs/docs (companion PR): https://github.com/seqeralabs/docs/pull/new/docs/wave-registry-troubleshooting

🤖 Generated with Claude Code

[christopher-hakkaart]

@MichaelTansiniSeqera - is this supposed to be in draft? Or is it okay to review and merge?