docs: document evolving specs in existing projects

[WOLIKIMCHENG]

Description

Closes #916.

Documents how existing projects can evolve specs over time using the persistence models introduced in the Spec Persistence Models guide. The update moves the brownfield workflow into a dedicated Evolving Specs guide and links to it from the README and SDD concepts page.

Scope:

• Adds docs/guides/evolving-specs.md with guidance for flow-forward, living spec, and flow-back workflows.
• Keeps Spec Kit project-file updates separate from feature artifact evolution.
• Updates README, SDD concepts, docs TOC, and DocFX content globs.
• Removes the brownfield workflow section from the upgrade guide.

Non-goals:

• No CLI behavior changes.
• No new specify update command.
• No template, script, or extension changes.

Testing

• git diff --check
• python -m json.tool docs/docfx.json
• Parsed docs/toc.yml and checked updated local docs links/anchors

[mnriem]

Thanks for tackling this — the gap you're addressing is real, and the step-by-step workflow is useful.

Since this PR was opened, #2856 landed on main introducing the spec persistence models page (flow-back, flow-forward, living). Your brownfield guidance should build on that vocabulary rather than existing independently.

Requested changes:

1. Rebase onto main to pick up docs: document spec persistence models #2856.

2. Move the new content out of upgrade.md into docs/guides/evolving-specs.md. The upgrade guide is about refreshing Spec Kit tooling — evolving your specs is a different workflow.

3. Structure the guide around the three persistence models. One document, three sections — each with clear steps for that model:

   • Flow-forward: create a new feature directory, run the standard specify → plan → tasks → implement flow.
   • Living spec: edit spec.md first, then regenerate plan.md and tasks.md from it.
   • Flow-back: edit wherever the insight lands, then reconcile the artifact set.

4. Drop or consolidate the sdd.md addition — that file already links to the persistence-models page. A single sentence there pointing to your new guide is enough.

5. Keep the README.md addition but update the link target to the new guide path.

6. Add the new page to docs/toc.yml under a Development or Guides section.

The scope stays docs-only and the core content you wrote (the step-by-step workflow) remains — it just gets framed per-model so readers with different conventions get tailored guidance.

[WOLIKIMCHENG]

Thanks for the detailed review. I've updated the PR to build on the spec persistence models and moved the brownfield guidance into docs/guides/evolving-specs.md. The guide is now organized around flow-forward, living spec, and flow-back, with the README, SDD concepts page, docs TOC, and DocFX content globs updated accordingly.