Update links in Resources section

[huijing]

Closes #334

This PR fixes the "Page not found" issue with the 2 links in the Resources section. Upon further inspection, it seems that the first 3 links are incorrect and this has been rectified as well.

FYI, the specification, although accessible at https://webmonetization.org/specification.html, is not actually part of the SPA pages generated by Docusaurus, therefore we need to utilise the escape hatch of pathname:// to resolve this (see https://docusaurus.io/docs/advanced/routing#escaping-from-spa-redirects)

🎩 Top-hatting instructions (i.e. human testing that things are not broken)

1. Pull this branch
2. Go to http://localhost:3000/ and scroll to the resources section. Verify that all links are working correctly.

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[sublimator]

checked as well, LGTM