Readme: Reorganize, add VS Code and performance table notes

[JesseHerrick]

• Reorganizes headings so that we focus on Quick Start first, moving the Why and Performance sections below.
• Adds a note to the VS Code section about potentially needing to set Dexter as your default formatter for Elixir.
• Clarifies performance table: "Init" was vague and could have implied LSP startup time rather than cold indexing time.

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Note

Low Risk
Low risk documentation-only changes; no runtime or API behavior is modified.

Overview
Moves the README’s motivation and benchmarks ("Why build another LSP?" and "Performance") below the setup sections, and updates the table of contents to match.

Clarifies VS Code format-on-save setup by noting you may need to set Dexter as the default Elixir formatter, and renames the performance table’s init row to "Cold first-time index" to avoid ambiguity.

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