Fix dead links

content/documentation/general/fediverse/index.md

content/service/dns/_index.md

content/service/feeds/_index.md

content/service/gitlab/_index.md

content/service/mastodon/_index.md

content/service/matrix/_index.md

content/service/matrix/administration/_index.md

content/service/mobilizon/_index.md

content/service/nextcloud/_index.md

content/service/pix/_index.md

content/service/video/_index.md