Add automatic generation of sitemap

[jakubgs]

We need a proper sitemap. Here's an example

Some requirements:

• Include one sitemap per blog category, if your site has distinct sections.
• Exclude image URLs from sitemaps (they add noise and offer little SEO value).
• Submit your sitemap in Google Search Console.

For more context see Notion doc:
https://www.notion.so/Onsite-Basic-Strategy-23f8f96fb65c814c9190f3f0f94814ee?p=23f8f96fb65c8198a900d2c761105a2d&pm=s

[phift]

Fixed this in here. At least as a first version. #83