docs: add contributing guide and partner plugin testing guide

[glaubersilva]

Summary

Adds developer documentation split out from the declined rebranding PR: #175

Despite the declination, the contributor workflow and partner-plugin testing guides are still valuable on their own. This PR contains documentation only: no PHP/JS code, no branding changes, no built assets.

New docs

• CONTRIBUTING.md — contributor quick start:
  • Primary local workflow via harbor-dev-tools (composer harbor:watch + npm run build/start for frontend work)
  • Prerequisites table (Bun 1.3.x requirement and why, Node, Composer, PHP)
  • Documentation index and release/PR expectations (build/ handling, composer release:prep)
• docs/guides/partner-plugin-testing.md — advanced guide: testing local Harbor changes inside a real partner plugin via a Composer path repository:
  • Why "stellarwp/harbor": "*@dev" and "symlink": false are required
  • Iteration cycle (composer update once, composer install per iteration) and why composer strauss alone fails
  • Environment-specific setup: native PHP (Herd/Valet/MAMP/WAMP), Lando/Docker, and wp-env — including the bind-mount + safe.directory steps when Harbor lives outside the site root
  • Cleanup steps to restore the partner plugin

Updated docs

• docs/guides/testing.md — replaces the outdated sample-plugin fixture workflow with harbor-dev-tools: layout, fixture key guidance (lwsw-unified-basic-2026 for the Software Manager UI), admin URLs, and the current fixture file resolution order (vendor-prefixed/stellarwp/harbor/tests/_data/)
• README.md — adds a Contributing section and links the new partner-plugin testing guide
• docs/harbor.md — adds the new guide to the documentation table
• .cspell.json — adds words used by the new docs (lando, mamp, wamp, docroot, appserver, lightningcss)

Notes for reviewers

• These docs were adapted to reflect main as-is: all references tied to the rebranding (e.g. the nexcess-software-manager admin slug) were removed — the admin URL table in testing.md points to the current lw-software-manager slug only.
• markdownlint and cspell pass on all changed files.

Test plan

• npm run lint:md and bun run check:spelling pass
• Follow CONTRIBUTING.md quick start on a clean checkout (harbor-dev-tools + composer harbor:watch)
• Spot-check cross-links between README, docs/harbor.md, CONTRIBUTING.md, and the guides