There's no link to download the SDK on http://api.libreoffice.org or http://api.libreoffice.org/docs/install.html Searching Libreoffice SDK in google brings you to those pages.
Download of SDK and source are in the lower part of the official download page http://www.libreoffice.org/download/download/ But I agree, there should be a hint or a direct link to it in http://api.libreoffice.org or http://api.libreoffice.org/docs/install.html.
Michael Stahl committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/core/commit/?id=7cecf138815770ade61c090efad2258f5baf5c9a tdf#107785 odk: document where to download the SDK It will be available in 6.0.0. The patch should be included in the daily builds available at http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More information about daily builds can be found at: http://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.
i've added a link that should show up on http://api.libreoffice.org/docs/install.html at some undetermined point in the future when the content on the server is updated.
Michael Stahl committed a patch related to this issue. It has been pushed to "libreoffice-5-4": http://cgit.freedesktop.org/libreoffice/core/commit/?id=20989f6c206619e527d57d79d5b8b7c8ef3fe70d&h=libreoffice-5-4 tdf#107785 odk: document where to download the SDK It will be available in 5.4.2. The patch should be included in the daily builds available at http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More information about daily builds can be found at: http://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.