feat(v2): allow custom heading component

[endiliey]

Motivation

Rather than hardcoding/ transforming the header automatically in html, we should allow composition style. This will allow user to modify the heading as they like. Example: if they dont want hash icon

previously

docusaurus/packages/docusaurus-mdx-loader/src/rehype/linkHeadings.js

Lines 8 to 56 in 55ef751

	const visit = require('unist-util-visit');
	const createAnchor = id => ({
	type: 'element',
	tagName: 'a',
	properties: {
	ariaHidden: true,
	className: 'anchor',
	id,
	},
	});
	const createLink = id => ({
	type: 'element',
	tagName: 'a',
	properties: {
	ariaHidden: true,
	className: 'hash-link',
	href: `#${id}`,
	},
	children: [
	{
	type: 'text',
	value: '#',
	},
	],
	});
	const headings = ['h1', 'h2', 'h3', 'h4', 'h5', 'h6'];
	const visitor = node => {
	const {properties} = node;
	if (!properties || !properties.id || !headings.includes(node.tagName)) {
	return;
	}
	node.children.unshift(createLink(properties.id));
	node.children.unshift(createAnchor(properties.id));
	delete properties.id;
	};
	const transformer = node => {
	visit(node, 'element', visitor);
	};
	const plugin = () => transformer;
	module.exports = plugin;

For now i make it the same format as before, feel free to tweak in new pr

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

• anchor and hash link still works. same format as before but now easily swizzleable
  

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[yangshun]

Would people accidentally cause their headings on blog to break if they only want to modify the headings on docs?

[endiliey]

@yangshun

technically theme classic use the same mdxcomponents. if they want different, swizzle the blog layout to use different comp.

anw this pr is better than previous, previously we alrd transform the heading in html 😂