docs: refresh signal library links

[whg517]

Summary

• replace two dead Blinker links from pythonhosted.org with the current stable documentation
• upgrade all six PyDispatcher SourceForge references to HTTPS, including the attribution inside the example docstring
• keep the document content and examples otherwise unchanged

Validation

• lychee 0.24.2 on signal_decouple.md: 11 links checked, 0 errors, 0 redirects
• markdownlint-cli2 0.23.2: 30 Markdown files, 0 issues
• MkDocs strict build in a clean Python 3.12 environment
• confirmed no legacy Blinker or HTTP PyDispatcher URLs remain
• git diff --check

References

• https://pypi.org/project/blinker/
• https://blinker.readthedocs.io/en/stable/
• https://pydispatcher.sourceforge.net/