[Yes, I know there is a far simpler method.]
curl -s 'https://mastodon.gougere.fr/.well-known/webfinger?resource=bortzmeyer@mastodon.gougere.fr' | \
jq '.links[] | if (.type == "application/atom+xml") then .href else "" end'
""
"https://mastodon.gougere.fr/users/bortzmeyer.atom"
""
""
""