sezero authored Nov 14, 2019 and Erik de Castro Lopo committed Nov 15, 2019

- document version requirements in the top-level file.
- remove incorrect version requirements from sub-dirs.
- set min. required version to 3.5 in top-level file.
- set minimun required version to 3.9 under docs/ .
- make documents building an option (on by default.)
  so, the tree can be built using cmake-3.5 using
 -DBUILD_DOCS=0 on the command line.