docs: unify wiki content into mdbook under development/

[bobluppes]

Summary

• Copies all pages from the GitHub wiki (Architecture, Contributing, Guides, home, sidebar) verbatim into docs/src/development/, wired into docs/src/SUMMARY.md under a new "Development" section — so contributor docs live alongside the existing user docs in a single mdbook site.
• The wiki itself is left intact for now; it can be removed separately.
• Updates README.md, CONTRIBUTING.md, and the issue templates to link at the new mdbook pages instead of the GitHub wiki.
• Rewrites the internal cross-links within the copied pages (e.g. links to "code of conduct", "development setup") to point at their new relative mdbook locations instead of the old wiki URLs.

Test plan

• mdbook build succeeds with no warnings
• Verified via diff that all copied pages are byte-identical to the original wiki source before link rewriting
• Confirmed no remaining github.com/bobluppes/graaf/wiki references anywhere in the repo

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.71%. Comparing base (e64eb61) to head (07fd7db).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #401   +/-   ##
=======================================
  Coverage   99.71%   99.71%           
=======================================
  Files          60       60           
  Lines        2823     2823           
  Branches      151      151           
=======================================
  Hits         2815     2815           
  Misses          8        8           

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