Add markdown options to AuthorOptions and add example usage

[Yuhanawa]

#185

export default defineTheme({
	// ..
	markdown: ({ config, astroConfig }) => ({
		remarkPlugins: [config.toc && [remarkToc, { heading: "toc", maxDepth: 3 }]],
	}),
});

or

	markdown: {
	      remarkPlugins: ({ config, astroConfig }) => [config.toc && [remarkToc, { heading: "toc", maxDepth: 3 }]]);
        }

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