Refactor documentation (#401)

Refactor documentation (#401)

* Refactor documentation

 - The getting started instructions from the README are moved to a
 dedicated doc/ page

 - The installation instruction now have toolchain install info

 - publishing.md is populated from the content of
 alire-index/CONTRIBUTING.md

 - policies.md is populated from the content of alire-index/POLICY.md

 - Some README platform info are updated

* Fix out of date info

* doc/getting-started.md: add a section on setenv

* doc/getting-started.md: add a section about msys2

Co-authored-by: Alejandro R Mosteo <amosteo@unizar.es>