Fix HTML validation issues on templates

Follow-up from #118 (comment 2081368103)

Let's fix these HTML issues:

• Homepage: https://validator.w3.org/nu/?doc=https%3A%2F%2Fnew.docs.gitlab.com%2F
• Interior page template: https://validator.w3.org/nu/?doc=https%3A%2F%2Fnew.docs.gitlab.com%2Fdevelopment%2Fdocumentation%2Fstyleguide%2F