Improve the installation documentation

[kit-ty-kate]

While reviewing #6367 i realized the installation documentation is outdated and would require constant update not to be.

This PR does:

• Replace the individual badge for each distribution&version by repology's vertical-allrepos with its exclude_unsupported=1 option (see https://repology.org/project/referencing/badges) to only show the distribution version not EoL.
• Remove Exherbo from the list of supported distribution as it only provides the 11 years out-of-date opam 1.1.1
• Add Alpine Linux to the list
• Merge the Debian and Ubuntu steps
• Remove mentions of CentOS and RHEL which do not have the opam package
• Remove the Distribution.md file which is extremely out-of-date, redundant with the new automatically generated repology badge and requires a huge effort to maintain

as well as some general fixes and improvements