Add more implementations

Instead of just linking to the reference implementation, add an
implementations page which links to TUF implementations (libraries) and
TUF systems.

Signed-off-by: Joshua Lock <joshuagloe@gmail.com>

Author: joshuagl

config.toml

@@ -108,9 +108,9 @@ weight = 5
 
 
 [[menu.main]]
-name = "Reference implementation"
+name = "Implementations"
 parent = "getting-started"
-url = "https://github.com/theupdateframework/python-tuf"
+url = "/implementations"
 weight = 6
 
 [[menu.main]]

content/implementations.md

@@ -0,0 +1,33 @@
+---
+title: Implementations
+---
+
+TUF provides a framework for integration of the security properties into new and existing content delivery systems.
+
+While some [adoptions](/adoptions) integrate TUF by implementing the framework
+from scratch, others start from either a TUF [implementation](#implementations)
+or from a TUF [system](#systems).
+
+This page lists open source implementations of TUF which can be used as
+building blocks for any TUF adoption.
+
+## Implementations
+
+TUF implementations provide libraries implementing the primitives and algorithms, such as the detailed client workflow, in the specification.
+
+* [python-tuf](https://github.com/theupdateframework/python-tuf) reference
+  implementation
+* [go-tuf](https://github.com/theupdateframework/go-tuf/)
+* [tuf-js](https://github.com/theupdateframework/tuf-js)
+
+## Systems
+
+TUF systems often build on an implementation and provide an opinionated implementation of TUF designed to solve a for a specific use-case.
+
+* [Repository Service for TUF (RSTUF)
+](https://repository-service-tuf.readthedocs.io/en/stable/) is a TUF repository
+designed to integrate into an existing community package repository with an
+established artifact storage and delivery system.
+* [tuf-on-ci](https://github.com/theupdateframework/tuf-on-ci/) is a TUF
+repository and signing tool designed to operate on a CI system and guide
+signing events through Git forge workflows.