docs, librex: invert README symlink direction

This allows the README to be rendered by default in both the docs and
the librex directories on GitHub.

Signed-off-by: Jinghao Jia <jinghao7@illinois.edu>

Author: jinghao-jia

docs/librex.md

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+# LIBREX
+
+`librex` serves as the loader library for Rex programs -- similar to what
+`libbpf` is to eBPF programs -- but in a simpler way.
+
+### APIs
+
+`librex` only contains the code that loads Rex programs from ELF binaries,
+other operations (e.g., attachment, map manipulation, etc) are delegated to
+the existing code in `libbpf`.
+
+Therefore, the library only exposes the following simple APIs:
+```c
+struct rex_obj *rex_obj_load(const char *file_path);
+struct bpf_object *rex_obj_get_bpf(struct rex_obj *obj);
+void rex_set_debug(int val);
+```
+
+The `rex_obj_load` function loads the Rex programs from the ELF binary
+identified by the `file_path` argument into the kernel. It returns a
+pointer to the corresponding `rex_obj` that encapsulates information about
+the loaded programs and created maps. If there is an error, a null pointer
+is returned.
+
+The `rex_obj_get_bpf` returns a pointer to the equivalent `bpf_object` of
+the `obj` argument, which can be subsequently passed to `libbpf` APIs. If
+there is an error, a null pointer is returned.
+
+**Note**: The returned pointer from both functions above are **non-owning**
+pointers, which means the caller of these function should not try to
+`free`/`realloc`/`delete` the pointer. The ownership of the pointers is
+managed by `librex` internally and will be automatically freed after the
+program terminates.
+
+The `rex_set_debug` function can be used to toggle the internal logging
+mechanism of `librex` (with `(bool)val` determining whether logging is
+enabled). This will most likely be helpful during debugging.
+
+### Build
+
+Building `librex` requires a `c++23` compatible compiler and the `mold`
+linker. With dependencies installed, simply invoke the Makefile:
+
+```bash
+make
+```

docs/librex.md

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+../docs/librex.md