[sofiabordagorry]

What does this PR do?

Closes #2232

hexdocs adds "retired": true to versionNodes entries for retired versions (hexpm/hexdocs#95). This PR makes ExDoc show them as vX.Y.Z (retired) in the sidebar version dropdown (as suggested here)

It also documents the optional latest and retired keys in the versionNodes docs

How to test

Run mix docs and add a doc/docs_config.js like:

var versionNodes = [
  {version: "v0.41.0", url: "#", latest: true},
  {version: "v0.40.4", url: "#", retired: true},
  {version: "v0.40.3", url: "#"}
]

Then open doc/index.html: the dropdown shows v0.40.4 (retired).

[github-actions]

📦 Docs artifacts are ready: https://github.com/elixir-lang/ex_doc/actions/runs/38321718157/artifacts/11794146314

[josevalim]

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