C API for Nix

1st chunk of `doc/manual/source/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](development/documentation.md#c-api-documentation), including how to build it locally.

Title: C API for Nix

Summary

Nix offers a C API that is currently under development with the goal of becoming stable. Resources for exploring the API include documentation, a Matrix room for discussion, the stabilization milestone, related pull requests and issues, and contribution guidelines for documentation.