* docs: add Prerelease editions install guidance (Next/Nightly/Insiders/Preview)
Documents the existing <RUNTIME>_CONFIG_DIR override pattern for users on
prerelease runtime editions (Windsurf Next, Cursor Nightly, VS Code Insiders,
Codex preview, JetBrains EAP, etc.) and explicitly states they are best-effort
and not separately tested under release CI — consistent with the free-string
runtime policy in #2517.
Resolves the discoverability gap behind issue #3161 without enumerating each
prerelease channel as a named runtime. Future "add <runtime>-next/-nightly"
requests can be redirected to the new section.
Closes#3172
* chore(changeset): set PR number for prerelease docs fragment
