* Bump package version to 5.1.0

* Documentation wrap-up (incl. package description update)

* Added link to API docs in landing page

* Fixed misplaced landing page table row

* Have to use ubuntu18 image with gcc9 for CI

* Install gcc-9 and g++-9 for sdk build job on Ubuntu18

* Install gcc-9 and g++-9 for UTR test jobs

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Co-authored-by: Sebastien Duverne <sebastien.duverne@unity3d.com>