[WEBSITE] Cleanup the fetch_documentation.sh script

Summary: The script had various fixes due to different behaviors between versions. All the special cased versions are now obsolete and the code can benefit from a good cleanup.

Test Plan:

./script/fetch_documentation.sh

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9875