Add EOL process doc

### What would you like to be added?

Related to `release.md`, we should also have a process doc for how versions are EOL'd. This is a little more than we want to put into release.md itself, I think, but that file would link to this one.

### Why is this needed?

Currently we don't have a doc for this at all.

Start by reading release.md to understand the existing release documentation and where a link to the new process belongs. Document how etcd versions are EOL'd, using the project's established release process as guidance, and consider the work done when the standalone document is complete and release.md links to it.

Written by the indexing model from the issue text.