docs: fix broken links in website

directory URLs no longer work by default, and site_urls config option has been made compulsory config option since version 1.2 of mkdocs and if site_urls option is not defined then use_directory_urls is set to false leading to broken links

mkdocs Release Notes: https://www.mkdocs.org/about/release-notes/#backward-incompatible-changes-in-12

PS: It would be great to make the site_url option dynamic, but I don't know how I can do that. Your suggestions are welcome.

Fixes: #317 (closed)