docs: remove unavailable sample source links

[whg517]

Summary

• remove the broken Word Count source link and describe the self-contained walkthrough instead
• remove the unavailable file2mongo source link while retaining the example name and layout
• clarify that file2mongo imports file data into MongoDB

Rationale

Both linked branches now return 404. The recoverable Word Count history uses Pipenv, setup.cfg, and a flat package layout, while the current guide uses Poetry and src/; presenting that snapshot as the complete current example would be misleading. No verifiable file2mongo source revision remains in the public refs or repository history.

Validation

• git diff --check
• npx --yes markdownlint-cli2@0.23.2 --config .markdownlint.yml '**/*.md'\n- clean Python 3.12 environment: python -m mkdocs build --strict\n- independent read-only review