docs: add repository-wide feature and operations guides

[drunkcoding]

Summary

• establish a layered documentation hub, changelog, model compatibility matrix, and focused configuration, environment, serving, multi-GPU, troubleshooting, and benchmark guides
• update DFlash, Qwen3.5, GLM-5.2, DeepSeek-V4, and contributor architecture documentation to match current code and validation boundaries
• add documentation-impact checks to the PR template, contribution guide, and release process, with recovered design and implementation plan artifacts

Documentation impact

• README discovery and navigation updated
• Authoritative feature, config, API, model, architecture, and benchmark guides updated
• CHANGELOG Unreleased updated
• No documentation impact

Validation

• pytest -q tests/python/dflash -m "not gpu" — 264 passed, 3 GPU-gated skipped
• focused config/model/serving/watchdog/distributed suite — 93 passed
• local Markdown links — 147 links, 37 anchors, 0 errors
• python -m compileall -q examples benchmarks
• git diff --check origin/feat/dflash-tracks-abcd...HEAD
• five-area final review: goal, QA, quality, security, and repository context all passed

Notes

• This PR intentionally targets feat/dflash-tracks-abcd; the docs describe capabilities in that feature stack and should not land independently on main.
• Markdown LSP is unavailable in the current environment; structural link, anchor, path, and command checks were used instead.
• Two watchdog integration fixture failures reproduce unchanged on the base branch; no runtime or test code is modified here.