fix: escape URLs in sitemap

[pawamoy]

Issue zensical/zensical#772.

Example sitemap after this change:

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
      <url>
        <loc>http:&#x2f;&#x2f;localhost:8000&#x2f;</loc>
      </url>
      <url>
        <loc>http:&#x2f;&#x2f;localhost:8000&#x2f;markdown&#x2f;</loc>
      </url>
      <url>
        <loc>http:&#x2f;&#x2f;localhost:8000&#x2f;test&amp;test&#x2f;test&#x2f;</loc>
      </url>
</urlset>

This fixes the instant preview in the mentioned issue.

[squidfunk]

Superseded by zensical/zensical#773. MkDocs uses percent-encoding in URLs, which is the canonical way to normalize URLs, and we were just missing the percent-encoding of & characters.