Fix #65: Adds site_url in the config to fix broken 404 page

Description

This PR adds site_url in the config to fix the broken 404 page reported in !65 (merged). [Reason for the changes: Debugging notes]

Although it's not a good idea to have the site URL in the config file as the site URL can be different based on where it's deployed but considering mkdocs generates static pages and won't be able to generate pages dynamically one has to follow this pattern as explained here.

JIRA tickets: https://tasks.opencraft.com/browse/SE-5386

Testing instructions:

Testing locally: Unable to test these changes locally as the local server overrides the site_url value.

Testing in production:

• Try to access an incorrect handbook page: https://handbook.opencraft.com/en/latest/invalid-page
• Page should load with a 404 message.

Reviewers:

• @pkulkark
• @FoxyFoxie