Update "Setup your docs environment" instructions to match current Hugo Modules layout

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Description

The contribution guide's setup steps pointed at a themes/docsy submodule that no longer exists and ran npm commands from the wrong directory, causing ENOENT/missing-directory errors for anyone following them from a fresh clone. The theme is now pulled in via Hugo Modules, and docs/package.json exposes npm start (not npm run dev).

• docs/content/contributing/contributing-docs/index.md (Local machine setup):
  • Removed git submodule update --init --recursive and the cd themes/docsy && npm install step
  • Corrected working directory to docs/docs; use npm ci then npm start
  • Added note that Docsy is resolved automatically as a Hugo Module
  • Upmerge note now references the docsy Hugo Module version (docs/go.mod/docs/go.sum) instead of a submodule
• docs/README.md: replaced both npm run dev references (no such script) with npm start

Setup now reads:

cd docs/docs
npm ci
npm start   # http://localhost:1313/

No remaining references to npm run dev, themes/docsy, or git submodule update across the docs.

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