chore: add docs for new custom domains service

[r-birkner]

Thank you for your contribution to the IC Developer Portal. This repo contains the content for https://internetcomputer.org and the ICP Developer Documentation, https://internetcomputer.org/docs/.

If you are submitting a pull request for adding or changing content on the ICP Developer Documentation, please make sure that your contribution meets the following requirements:

• [ ] Follows the developer docs style guide.
• [ ] Follows the best practices and guidelines.
• [ ] New documentation pages include document tags.
• [ ] New documentation pages include SEO keywords.
• [ ] New documents are in the .mdx file format to support the previous two components.
• [ ] New documents are registered in /sidebars.js, otherwise, it will not appear in the side navigation bar.
• [ ] Make sure that the .github/CODEOWNERS file is contains any new directories or specific documents that you added that should be reviewed by a specific teamm.