docs: document stdlib path override in Zed setup

[jamilabreu]

Adds a short note to the Zed setup section covering the Dexter: could not detect Elixir stdlib... startup warning: how to point Dexter at a stdlib source tree via the stdlibPath initialization option (or DEXTER_ELIXIR_LIB_ROOT via lsp.dexter.binary.env).

Context: I hit this warning in Zed with a Homebrew-installed Elixir. Homebrew's elixir ships only compiled .beam files — no .ex sources (find $(brew --prefix elixir) -name "*.ex" comes back empty) — so auto-detection correctly rejects it (the elixir/lib/enum.ex check in internal/stdlib/stdlib.go), and pointing DEXTER_ELIXIR_LIB_ROOT at the Homebrew lib dir silences the warning without enabling stdlib navigation. What worked was a checkout of elixir-lang/elixir at the matching tag. Since the README's Zed section didn't mention either override, this documents them where Zed users will look.

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Note

Low Risk
Documentation-only change with no runtime or configuration behavior changes.

Overview
Extends the Zed editor setup in the README with guidance for Dexter’s “could not detect Elixir stdlib” startup warning, especially when Elixir comes from Homebrew (no .ex stdlib sources).

The new section shows how to set stdlibPath under lsp.dexter.initialization_options, and notes the equivalent DEXTER_ELIXIR_LIB_ROOT override via lsp.dexter.binary.env, pointing users at a matching Elixir source tree (e.g. an elixir-lang/elixir checkout) for real stdlib navigation.

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[JesseHerrick]

Thanks very much for the contribution @jamilabreu!