gui, man, authors: Update docs, translations, and contributors

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gui/default/assets/lang/lang-eu.json

gui/default/assets/lang/lang-pl.json

gui/default/assets/lang/lang-pt-BR.json

gui/default/syncthing/core/aboutModalView.html

man/stdiscosrv.1

man/strelaysrv.1

man/syncthing-bep.7

man/syncthing-config.5

man/syncthing-device-ids.7

man/syncthing-event-api.7

man/syncthing-faq.7

man/syncthing-globaldisco.7

man/syncthing-localdisco.7

man/syncthing-networking.7

man/syncthing-relay.7

man/syncthing-rest-api.7

man/syncthing-security.7

man/syncthing-stignore.5

man/syncthing-versioning.7