Revert "[WEBSITE] Remove relative link conversion in the release notes"

Summary:
This reverts commit 98e2d8cfe362dcc2f3d8066bf66153b8b351ad28.

The jekyll plugin that converts the links doesn't run on the release page, so we still need to convert the links.

Test Plan:

./scripts/fetch_documentation.sh
bundle execy jekyll serve

Check the release page link for 0.26.7 now works.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12696