Port C API docs to Meson (#10936)

* Port C API docs to Meson

* don't cross-compile the docs

src/external-api-docs/README.md

@@ -0,0 +1,121 @@
+# Getting started
+
+> **Warning** These bindings are **experimental**, which means they can change
+> at any time or be removed outright; nevertheless the plan is to provide a
+> stable external C API to the Nix language and the Nix store.
+
+The language library allows evaluating Nix expressions and interacting with Nix
+language values. The Nix store API is still rudimentary, and only allows
+initialising and connecting to a store for the Nix language evaluator to
+interact with.
+
+Currently there are two ways to interface with the Nix language evaluator
+programmatically:
+
+1. Embedding the evaluator
+2. Writing language plug-ins
+
+Embedding means you link the Nix C libraries in your program and use them from
+there. Adding a plug-in means you make a library that gets loaded by the Nix
+language evaluator, specified through a configuration option.
+
+Many of the components and mechanisms involved are not yet documented, therefore
+please refer to the [Nix source code](https://github.com/NixOS/nix/) for
+details. Additions to in-code documentation and the reference manual are highly
+appreciated.
+
+The following examples, for simplicity, don't include error handling. See the
+[Handling errors](@ref errors) section for more information.
+
+# Embedding the Nix Evaluator{#nix_evaluator_example}
+
+In this example we programmatically start the Nix language evaluator with a
+dummy store (that has no store paths and cannot be written to), and evaluate the
+Nix expression `builtins.nixVersion`.
+
+**main.c:**
+
+```C
+#include <nix_api_util.h>
+#include <nix_api_expr.h>
+#include <nix_api_value.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+// NOTE: This example lacks all error handling. Production code must check for
+// errors, as some return values will be undefined.
+
+void my_get_string_cb(const char * start, unsigned int n, void * user_data)
+{
+    *((char **) user_data) = strdup(start);
+}
+
+int main()
+{
+    nix_libexpr_init(NULL);
+
+    Store * store = nix_store_open(NULL, "dummy://", NULL);
+    EvalState * state = nix_state_create(NULL, NULL, store); // empty search path (NIX_PATH)
+    Value * value = nix_alloc_value(NULL, state);
+
+    nix_expr_eval_from_string(NULL, state, "builtins.nixVersion", ".", value);
+    nix_value_force(NULL, state, value);
+
+    char * version;
+    nix_get_string(NULL, value, my_get_string_cb, &version);
+    printf("Nix version: %s\n", version);
+
+    free(version);
+    nix_gc_decref(NULL, value);
+    nix_state_free(state);
+    nix_store_free(store);
+    return 0;
+}
+```
+
+**Usage:**
+
+```ShellSession
+$ gcc main.c $(pkg-config nix-expr-c --libs --cflags) -o main
+$ ./main
+Nix version: 2.17
+```
+
+# Writing a Nix language plug-in
+
+In this example we add a custom primitive operation (_primop_) to `builtins`. It
+will increment the argument if it is an integer and throw an error otherwise.
+
+**plugin.c:**
+
+```C
+#include <nix_api_util.h>
+#include <nix_api_expr.h>
+#include <nix_api_value.h>
+
+void increment(void* user_data, nix_c_context* ctx, EvalState* state, Value** args, Value* v) {
+    nix_value_force(NULL, state, args[0]);
+    if (nix_get_type(NULL, args[0]) == NIX_TYPE_INT) {
+      nix_init_int(NULL, v, nix_get_int(NULL, args[0]) + 1);
+    } else {
+      nix_set_err_msg(ctx, NIX_ERR_UNKNOWN, "First argument should be an integer.");
+    }
+}
+
+void nix_plugin_entry() {
+  const char* args[] = {"n", NULL};
+  PrimOp *p = nix_alloc_primop(NULL, increment, 1, "increment", args, "Example custom built-in function: increments an integer", NULL);
+  nix_register_primop(NULL, p);
+  nix_gc_decref(NULL, p);
+}
+```
+
+**Usage:**
+
+```ShellSession
+$ gcc plugin.c $(pkg-config nix-expr-c --libs --cflags) -shared -o plugin.so
+$ nix --plugin-files ./plugin.so repl
+nix-repl> builtins.increment 1
+2
+```