* Fix readme typos

* Mention dev/build.sh in readme

* alire_common.gpr: remove default host os to avoid mistakes on macOS

With the default being Linux, macOS users will not notice any issue until they try to use the executable.
Even then the issue will be obscure.

* Remove any implicit detection of the host OS

This should avoid misbuilds that mix Linux/macOS.

A bash function is now provided for detection of the OS in our scripts.

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

* Getting started: spelling fixes

* README: add beta info and build instructions

* 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* Fix out of date info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>

Fixes #299

Mention that we are ramping up towards public beta so changes to be expected.
Hide the installer until we have something minimally stable.
Updated installation instructions.
Remove the link to the paper that explains the self-compiling version.

Fixes #163

[skip ci]