Linkcheck fixes

[stevepiercy]

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📚 Documentation preview 📚: https://plone6--1837.org.readthedocs.build/

[stevepiercy]

gnu.org has been having issues for a while. They work fine most of the time.

(        glossary: line  536) broken    https://www.gnu.org/savannah-checkouts/gnu/gettext/manual/html_node/PO-Files.html - HTTPSConnectionPool(host='www.gnu.org', port=443): Max retries exceeded with url: /savannah-checkouts/gnu/gettext/manual/html_node/PO-Files.html (Caused by NewConnectionError('<urllib3.connection.HTTPSConnection object at 0x7fbd6c9d0cb0>: Failed to establish a new connection: [Errno 101] Network is unreachable'))