[website] There might be no relative link in the release notes

Summary: But the documentation fetching script will fail in this case. Just ignore the error and skip the useless conversion instead.

Test Plan:

./scripts/fetch_documentation.sh

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12895