This is a custom pip index.
Read more about how this works here.
The pages for the index are generated using a custom script Gorialis made that templates using Jinja2. You can see the source on the repository.
Gorialis styled the page originally based on how Apache automatically generates its directory indices, but they softened it up to make it a bit easier on the eyes, without adjusting the key components that allow pip to detect the packages from it.
To allow pip to download from this index, you can use the --extra-index-url flag, e.g.:
pip install discord.py[voice] --extra-index-url https://abstractumbra.github.io/pip/