[docs] Correction - GitHub Pages now supports HTTPS with custom domains

[andrewnicolalde]

The Publii docs say that GitHub Pages sites using custom domains don't support HTTPS, however this changed in 2018.

This section of the Publii GitHub Pages docs should be removed to reflect GitHub Page's support for HTTPS for sites using custom domains.

Relevant GitHub docs: https://docs.github.com/en/pages/getting-started-with-github-pages/securing-your-github-pages-site-with-https#about-https-and-github-pages

[bobmitro]

Thank you for the advice; the docs has been updated.