docs: edit some docs to adjust tone

[jderochervlk]

These documentation edits were generated by AI with the following prompts:

• Remove comparisons to other languages (especially JavaScript)
• Remove deprecated references
• Ensure the language manual doesn't read like a step-by-step guide
• Adjust the tone to be less casual

What changed

Overview page rewritten (overview.mdx): The "Comparison to JS" two-column table format (JavaScript | ReScript) was replaced with a standalone syntax reference. Each section now describes ReScript on its own terms rather than defining it relative to JavaScript.

Removed deprecated pages:

• browser-support-polyfills.mdx -- outdated ES5/TypedArray polyfill guidance
• build-external-stdlib.mdx -- the external-stdlib / @rescript/std documentation
• Corresponding path entries removed from Path.res

Deprecated config keys updated (build-configuration.mdx, build-monorepo-setup.mdx):

• bs-dependencies / bs-dev-dependencies renamed to dependencies / dev-dependencies (with a note that legacy keys are still accepted)
• bsc-flags renamed to compiler-flags (with the same deprecation note)
• external-stdlib section removed

Float operators modernized for v12+ across multiple files:

• +., -., *., /. replaced with unified operators +, -, *, /
• primitive-types.mdx updated to document that arithmetic operators work for both int and float

Tone and phrasing adjustments across ~20 files:

• Removed casual language (=P, =), thankfully, jokes about bosses)
• Removed "Since X.Y" version markers that dated the documentation
• Replaced sequential references ("the previous section", "a later section") with direct links
• Removed comparisons to TypeScript's DefinitelyTyped ecosystem
• Updated JS output examples to use native methods instead of Belt (Belt_Array.map to .map, Belt_Array.concatMany to native spreads)
• Changed "JS" abbreviations to "JavaScript" in several places for consistency

Files changed

Category	Files
Removed	browser-support-polyfills.mdx, build-external-stdlib.mdx
Major rewrite	overview.mdx
Config docs	build-configuration.mdx, build-monorepo-setup.mdx
Float operator updates	primitive-types.mdx, interop-cheatsheet.mdx, type.mdx, generalized-algebraic-data-types.mdx, extension_identity.mdx, language_labeled_argument.mdx, language_open.mdx
Tone/phrasing	introduction.mdx, array-and-list.mdx, async-await.mdx, control-flow.mdx, converting-from-js.mdx, editor-plugins.mdx, embed-raw-javascript.mdx, exception.mdx, external.mdx, function.mdx, generate-converters-accessors.mdx, import-from-export-to-js.mdx, interop-with-js-build-systems.mdx, jsx.mdx, libraries.mdx, module.mdx, record.mdx, shared-data-types.mdx, build-performance.mdx

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