Add upstream relationship documentation

[pbajjuri20]

Summary

• Add docs/upstream.md documenting Kiali's role as the community upstream project and its relationship to OSSM, related repositories, and release branches.
• Covers repository layout, OSSM branch mapping, contribution workflow, release/sync process, OSSMC compatibility notes, CI, and PR targets.

Test plan

• Documentation-only change; no code or CI impact
• Review links and branch mapping table for accuracy
• Confirm wording with maintainers (especially OSSM release alignment)

Made with Cursor

[jshaughn]

@pbajjuri20 I'm not sure what purpose this is serving. It seems like an OSSM-specific document inside of the docs folder. upstream.md is sort of vague. Can you better describe the benefit of this file?

[jmazzitelli]

meh - this is now yet another place we have to maintain compatibility matrix?

[pbajjuri20]

@pbajjuri20 I'm not sure what purpose this is serving. It seems like an OSSM-specific document inside of the docs folder. upstream.md is sort of vague. Can you better describe the benefit of this file?

We’re doing this task to create a standard, explicit upstream/downstream workflow doc (docs/upstream.md) across OSSM repos so contributors (and AI agents) don’t have to infer the process from scattered tribal knowledge.

For this ticket, the goal is to document things like:

where changes should be proposed first (upstream vs related repos),
PR/backport and sync expectations,
coding/process alignment with upstream conventions.
That improves consistency, reduces wrong-target PRs, and makes agent-assisted contribution flows safer and more predictable across repositories (OSSM-13926).