Add sitemap plugin

[devadvance]

Description

Adds the sitemap plugin to more easily index the site.

Required checklist:

• My code matches the existing code style of this project.
• One of these:
  • This is a new localization for existing content.
  • This is new content that is localized for all the locales that this project currently supports.
  • I described above how I plan to localize this content immediately after merging these changes.
  • Localization is not applicable to this change.
• I have tested my code and it successfully builds for GitHub Pages and validates as AMP.