docs: add Chaintable security policy and contributing guide

[littlehand26]

What

Replaces the community docs inherited from upstream with Chaintable-specific ones. Both documents follow the structure piloted and merged in Chaintable/go-ethereum#53:

1. Lead with the fork dichotomy — this repo is a fork (upstream client + the Chaintable pipeline tracer), so the first thing each document does is split the world in two:

• Upstream issue / chain-core change (reproduces on an unmodified upstream build): follow the upstream process — SECURITY.md links the upstream security channel; CONTRIBUTING points at upstream without deep links. This document does not apply.
• Pipeline layer issue / change (the pipeline tracer and its block-data output, Dockerfile/images, CI): follow our process.

2. "Our process" follows the shared Chaintable doc structure (Supported Versions / Reporting / Response Process / Disclosure / Scope; Getting Started / Development Workflow / Local Checks / Code Guidelines / Testing / Pull Requests / Commit Guidelines / Releases / License), adapted to this repo's toolchain, build targets, release stream, and license files.

Where the upstream copy of CONTRIBUTING.md lives under .github/, it is removed in the same commit: GitHub resolves community health files with .github/ taking precedence over the repository root, so keeping it would shadow the new root file.

Upstream's own policies are referenced by link rather than copied: security contact info (PGP keys, bounty scope) goes stale in a snapshot, the canonical upstream links are always current, and the original files remain available in git history.

Why

The inherited community files pointed all reports and contributions at upstream's process — misrouting anything about the pipeline layer, and saying nothing about where chain-level issues should go. Upstream issues follow upstream's logic; pipeline issues follow ours.

Part of the org-wide rollout across Chaintable public write-node forks.

Also updates the workflow_dispatch version-input example in release.yml to the current release tag scheme (v<base-version>-ct.N) — comment-level text only, no behavior change.