[0.17] Doc: cmake now joins parts of offline doc-Fixes

[sgrogan]

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• [x ] Branch re-based on latest master git pull --rebase upstream master
• [x ] Unit tests confirmed to pass by running ./bin/FreeCAD --run-test 0
• [? ] Commit message is well-written
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Fixes Failed build on Travis: https://travis-ci.org/FreeCAD/FreeCAD/builds/355423073?utm_source=github_status&utm_medium=notification

Create destination directory for concatenated file prior to writing file.
Concatenates the offline doc and writes to Build/doc directory so it's available with an non-installed build.
Only concatenates the file if BUILD_GUI=TRUE

[wwmayer]

Merged.