chore: correct options.development sections parsing (#1798)

## PR Checklist

- [x] Addresses an existing open issue: fixes #1797
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/create-typescript-app/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/create-typescript-app/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

Splits it out into a separate `readDocumentation` function.

💖

src/next/base.ts

@@ -17,6 +17,7 @@ import { readGuide } from "../shared/options/createOptionDefaults/readGuide.js";
 import { readPackageData } from "../shared/packages.js";
 import { tryCatchLazyValueAsync } from "../shared/tryCatchLazyValueAsync.js";
 import { AllContributorsData } from "../shared/types.js";
+import { readDocumentation } from "./readDocumentation.js";
 import { swallowError } from "./utils/swallowError.js";
 
 export const base = createBase({
@@ -135,13 +136,7 @@ export const base = createBase({
 			return contributions.contributors;
 		});
 
-		const documentation = lazyValue(async () =>
-			swallowError(
-				await take(inputFromFile, {
-					filePath: ".github/DEVELOPMENT.md",
-				}),
-			),
-		);
+		const documentation = lazyValue(async () => readDocumentation(take));
 
 		const nvmrc = lazyValue(
 			async () =>

src/next/readDocumentation.test.ts

@@ -0,0 +1,26 @@
+import { describe, expect, it } from "vitest";
+
+import { readDocumentation } from "./readDocumentation.js";
+
+describe("finalize", () => {
+	it("returns undefined when no .github/DEVELOPMENT.md exists", async () => {
+		const documentation = await readDocumentation(() =>
+			Promise.resolve(undefined),
+		);
+
+		expect(documentation).toBeUndefined();
+	});
+
+	it("filters known headings when .github/DEVELOPMENT.md exists", async () => {
+		const documentation = await readDocumentation(() =>
+			Promise.resolve(`# Development\nremoved\n\n## Unknown\n\nKept.\n`),
+		);
+
+		expect(documentation).toMatchInlineSnapshot(`
+			"## Unknown
+
+			Kept.
+			"
+		`);
+	});
+});

src/next/readDocumentation.ts

@@ -0,0 +1,37 @@
+import { TakeInput } from "create";
+import { inputFromFile } from "input-from-file";
+
+import { swallowError } from "./utils/swallowError.js";
+
+const knownHeadings = new Set([
+	"building",
+	"development",
+	"formatting",
+	"linting",
+	"testing",
+	"type checking",
+]);
+
+export async function readDocumentation(take: TakeInput) {
+	const existing = swallowError(
+		await take(inputFromFile, {
+			filePath: ".github/DEVELOPMENT.md",
+		}),
+	);
+	if (!existing) {
+		return undefined;
+	}
+
+	return existing
+		.split(/\n\n(?=##\s)/)
+		.filter((section) => !knownHeadings.has(parseHeading(section)))
+		.join("\n\n");
+}
+
+function parseHeading(section: string) {
+	return section
+		.split("\n")[0]
+		.replace(/^#+\s+/, "")
+		.trim()
+		.toLowerCase();
+}