Base: Describe kernel modules, syscalls, and programs

Base/usr/share/man/man7/kernel_modules.md

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+## Name
+
+Kernel Modules - runtime code loading for the kernel
+
+## Description
+
+Serenity's kernel supports loading modules at runtime. This functionality can
+be used to implement optional features (e.g. drivers), and speed up your
+development cycle.
+
+## Module format
+
+A kernel module is a regular ELF object file which must export several
+symbols. Any symbols it refers to will be resolved when it is loaded.
+
+### `module_name`
+
+This should be a string like `const char module_name[]` containing the name of
+the module. This is used to give the module a name in any informational
+contexts, but also to ensure that the module is not loaded twice by accident,
+and also used as a reference to unload the module later.
+
+### `module_init`
+
+This should be a function with the following signature: `void module_init()`.
+It will be called when the module is loaded.
+
+### `module_fini`
+
+This is optional, but if defined it should be a function with the following
+signature: `void module_fini()`. It will be called when the module is
+unloaded.
+
+## Example:
+
+```c++
+#include <Kernel/kstdio.h>
+#include <Kernel/Process.h>
+
+extern "C" const char module_name[] = "ExampleModule";
+
+extern "C" void module_init()
+{
+    kprintf("ExampleModule has booted!\n");
+
+    for (int i = 0; i < 3; ++i) {
+        kprintf("i is now %d\n", i);
+    }
+
+    kprintf("current pid: %d\n", current->process().sys$getpid());
+    kprintf("current process name: %s\n", current->process().name().characters());
+}
+
+extern "C" void module_fini()
+{
+    kprintf("ExampleModule is being removed!\n");
+}
+```
+
+## See also
+
+* [`modload`(1)](../man1/modload.md)
+* [`modunload`(1)](../man1/modunload.md)
+* [`module_load`(2)](../man2/module_load.md)
+* [`module_unload`(2)](../man2/module_unload.md)