Explain new simpler process for submitting sitemap to Google Search Console

[benhalpern]

Explain the changes

Admins can now be instructed to submit /sitemap-posts.xml as the default way to submit to Google Search Console if this is merged:

forem/forem#14857

They can optionally be instructed to submit monthly sitemaps if they want to get specific breakdowns month-over-moth, but this is more advanced functionality for very large Forems. This functionality has already been in place, but we did not have the more basic functionality in place.