docs: format source references as list items

[DwarfM42]

Summary

• Formats each existing Sources reference as an individual Markdown list item for clearer visual separation on GitHub.
• Preserves the existing source numbers, URLs, descriptions, ordering, and all Related tools / how this differs citation numbers.
• Limits the change to README layout; it changes no claim or reference target.

Validation

• Mechanical audit: all 7 Related tools / how this differs citations match their Sources entries.
• Internal Markdown links: valid.
• bash scripts/cargo-local.sh fmt --check
• bash scripts/cargo-local.sh test --all-targets --locked -- --nocapture
• bash scripts/cargo-local.sh check --all-targets --locked
• bash scripts/cargo-local.sh build --release --locked --bin perf-probe
• git diff --check