doc: add project processes documentation

[cdecker]

Summary

Adds doc/contribute-to-core-lightning/project-processes.md: an authoritative reference for how the project handles issues, pull requests, reviews and releases — and the rationale behind those processes. The goal is to have a document we can point to when explaining a triage action (why an issue was closed, why a PR is still Draft, why a reviewer hasn't looked at a branch yet), instead of re-explaining the reasoning each time.

It covers:

• Issue lifecycle: the Type::/Status::/Severity:: label taxonomy and the reasons we close issues (needs-info timeout, support questions, duplicates, out of scope, fixed at merge).
• Pull requests: the rebase model and its commit-hygiene consequences, the Draft-until-ready convention, and reproduction PRs (Type::Repro).
• Review & merge: reviewer assignment, one maintainer ACK + green CI.
• Releases: quarterly cadence and the branch-at-version-bump model, with rationale.
• Security issues: pointer to the security policy.

The page publishes automatically to https://docs.corelightning.org/docs/project-processes via the ReadMe sync once merged.

Marked as draft while the team reviews whether the described processes match everyone's understanding.

Checklist

• The changelog has been updated in the relevant commit(s) according to the guidelines. (Changelog-None, docs-only)
• Tests have been added or modified to reflect the changes. (N/A)
• Documentation has been reviewed and updated as needed.
• Related issues have been listed and linked, including any that this PR closes. (None)
• Important All PRs must consider how to reverse any persistent changes for tools/lightning-downgrade (N/A)

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