doc/manual: Add C API to menu

doc/manual/src/SUMMARY.md.in

@@ -112,6 +112,7 @@
   - [Store Path Specification](protocols/store-path.md)
   - [Nix Archive (NAR) Format](protocols/nix-archive.md)
   - [Derivation "ATerm" file format](protocols/derivation-aterm.md)
+- [C API](c-api.md)
 - [Glossary](glossary.md)
 - [Contributing](contributing/index.md)
   - [Hacking](contributing/hacking.md)

doc/manual/src/c-api.md

@@ -0,0 +1,16 @@
+# 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

doc/manual/src/contributing/documentation.md

@@ -207,8 +207,9 @@ or inside `nix-shell` or `nix develop`:
 # xdg-open ./outputs/doc/share/doc/nix/internal-api/html/index.html
 ```
 
-## C API documentation (experimental)
+## C API documentation
 
+Note that the C API is not yet stable.
 [C API documentation] is available online.
 You can also build and view it yourself: