Standardize external links using _config.yml variables

[namiroues]

We should standardize how external links (e.g., PyPI, GitHub repo, documentation site) are referenced in the documentation by pulling them from _config.yml instead of hard-coding them in Markdown files.

This will ensure that link updates only need to be made in one place, improving maintainability and consistency across the entire site.

Tasks:

1. Add/Confirm the following keys in _config.yml (e.g. pypi.project_url, github.repo_url)
2. Scan across the full repo for hard-coded links.
3. Replace them with Liquid references (e.g., {{ site.pypi.project_url }}).
4. Remove redundant references where needed.
5. Verify that all links render correctly after the change.

This issue is linked to this #458 (see this comment).