getLinks(html) {
        const cheerio = require('cheerio');
        const $ = cheerio.load(html);
        const urls = [];
        const siteUrl = this.siteUrl;

        for (const el of $('a').toArray()) {
            const href = $(el).attr('href');
            if (href) {
                let url;
                try {
                    url = new URL(href);

                    if (siteUrl && url.hostname === siteUrl.hostname) {
                        // Ignore links to the site's own domain
                        continue;
                    }

                    if (['http:', 'https:'].includes(url.protocol) && !urls.find(u => u.href === url.href)) {
                        // Ignore duplicate URLs
                        urls.push(url);
                    }
                } catch (e) {
                    // Ignore invalid URLs
                }
            }
        }
        return urls;
    }
@private
Get all external links in a HTML document.
Excludes the site's own domain.
@param {string} html
@returns {URL[]}