UEFI is the industry standards body that develops and distributes the UEFI, PI, and ACPI specifications. These specifications govern the firmware interfaces between OS, OEM/Device Manufacturer, and Silicon partner. This is a great site to download the industry specifications and if you are a member you can join working groups for future specifications.
Tianocore is an existing open source project. Their EDK2 repository is the basis for many/most UEFI implementations used on products today. It provides UEFI spec compliant code modules, supports industry standard hardware, and a multi-platform build environment. This is a great site to download specifications for the different file types and build process. It also has links to repositories that Project Mu tracks as "upstreams".
Great tool for creating documentation websites based on markdown. In fact it was used to generate this documentation.
Quick link for common markdown support.