doc/manual: Add C API to menu

doc/manual/src/c-api.md

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+# C API
+
+Nix provides a C API with the intent of [_becoming_](https://github.com/NixOS/nix/milestone/52) a stable API, which it is currently not.
+It is in development.
+
+See:
+- C API documentation for a recent build of master
+  - [Getting Started]
+  - [Index]
+- [Matrix Room *Nix Bindings*](https://matrix.to/#/#nix-bindings:nixos.org) for discussion and questions.
+- [Stabilisation Milestone](https://github.com/NixOS/nix/milestone/52)
+- [Other C API PRs and issues](https://github.com/NixOS/nix/labels/c%20api)
+- [Contributing C API Documentation](contributing/documentation.md#c-api-documentation), including how to build it locally.
+
+[Getting Started]: https://hydra.nixos.org/job/nix/master/external-api-docs/latest/download-by-type/doc/external-api-docs
+[Index]: https://hydra.nixos.org/job/nix/master/external-api-docs/latest/download-by-type/doc/external-api-docs/globals.html