fix: remove helia.io cname

[2color]

Description

• This PR removes the config which sets the custom domain used for GitHub pages. Historically, the docs were deployed to the helia.io domain, however, since we introduced a website for the docs, we no longer want this.
• When a custom domain is configured, Github will redirect from the github.io domain to the custom domain
• fixes Documentation is down #655

Note

I have also manually removed the custom domain from the site config to ensure this works.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if necessary (this includes comments as well)
• I have added tests that prove my fix is effective or that my feature works