docs: use relative internal links

[whg517]

Summary

• replace five internal links that hard-code the /pythonic-project-guidelines/ deployment prefix with source-relative Markdown paths
• preserve the Poetry, Stevedore, and Pytest anchors while allowing MkDocs to resolve them for any deployment base URL
• eliminate all absolute-link diagnostics from the strict MkDocs build

Validation

• markdownlint-cli2 0.23.2: 30 Markdown files, 0 issues
• MkDocs strict build in a clean Python 3.12 environment with no absolute-link diagnostics
• offline link and fragment check on the three generated pages: 232 links checked, 0 errors
• confirmed no hard-coded internal site-prefix links remain
• git diff --check