feat: add EL install instructions

[d1nuc0m]

Adds install instructions for Enterprise Linux (CentOS Stream, RHEL) - repositories include stable etcd release for an OpenStack installation

Should be integrated with #774

[jmhbnz]

Investigating these repositories they appear to contain etcd 3.4.14 as the most recent version?

https://mirror.stream.centos.org/SIGs/9-stream/cloud/x86_64/openstack-zed/Packages/e/

[ ] etcd-3.4.14-1.el9s.x86_64.rpm 2022-04-11 17:23 11M

3.4.14 is over three years old: https://github.com/etcd-io/etcd/releases/v3.4.14.

With this in mind my preference would be not to promote this method of installation.

As discussed in #774 lets promote brew as the alternative installation method on Linux for those that don't want to download a release on Github. This is known to be regularly maintained and currently ships etcd 3.5.12. https://formulae.brew.sh/formula/etcd.

[d1nuc0m]

Investigating these repositories they appear to contain etcd 3.4.14 as the most recent version?

Yes because these are designed to install OpenStack, and some components depend on a specific version of etcd

It is not the only repository though, PostgreSQL has its own RPM extra repo, that actually ships 3.5.10

[d1nuc0m]

@jmhbnz I've added a repository list, PostgreSQL's one includes 3.5.12