Merge branch 'doc_update' into 'develop'

Updating the documentation in preparation for OTB 8.0.0-alpha1

See merge request !780
2 jobs for 2141-fails-to-build-with-dotb_use_siftfast-off-and-dbuild_testing-on in 1 minute and 50 seconds (queued for 1 second)
Status Job ID Name Coverage
  Precheck
passed #38137
fast-build

00:01:50

 
  External
passed cdash:ubuntu-18.04-fast