[SymForce] Get cc_sym docs to generate correctly

**Why this PR:**
We already have sphinx generate docs for everything in symforce. The problem, however, is sphinx will only generate docs for `cc_sym.py`, which merely re-exports the contents of `cc_sym.so`, leaving all of the contained classes and functions undocumented.

**The core fix:**
To fix this, nothing needs to be done for `make docs_apidoc`, as that seems to only generate `.rst` files noting that docs need to be generated for `cc_sym`. Instead, I just changed the file extension of `cc_sym.py` so sphinx will ignore it, and then copied in `cc_sym.so` so sphinx will use that module to generate the docs for `cc_sym` instead.

**Note on getting the path of cc_sym.so:**
To find `cc_sym.so`, I used the environment variable `PY_EXTENSION_MODULE_PATH`, and defaulted it to `build/pybind` so it works with the default settings. This is brittle though, as if one wants to change the location where the files are built, the `PY_EXTENSION_MODULE_PATH` won't be updated.

It would better to instead use the cmake variable `SYMFORCE_PY_EXTENSION_MODULE_OUTPUT_PATH`, which is the source of truth for where `cc_sym.so` will get built, or perhaps `symforce.path_util.cc_sym_install_dir()`, which gets its value from the cmake variable, but there isn't an easy way to do that with our handwritten `Makefile` that I know of.

GitOrigin-RevId: 8e7910920d8de6a47af5173ab6e2eb7470bf4579

Author: bradley-solliday-skydio

Makefile

@@ -165,9 +165,17 @@ docs_apidoc:
 	doxygen docs/Doxyfile-gen-cpp
 	$(PYTHON) -m breathe.apidoc -o docs/api-cpp --project api-cpp $(BUILD_DIR)/doxygen-cpp/xml
 	$(PYTHON) -m breathe.apidoc -o docs/api-gen-cpp --project api-gen-cpp $(BUILD_DIR)/doxygen-gen-cpp/xml
+# The generated symforce.cc_sym.rst file says to generate docs for symforce.cc_sym. However, that file
+# only rexports cc_sym, and consequently the actual contents of cc_sym are not documented. We replace
+# symforce.cc_sym.rst with a copy which is the same except it says to generate docs for cc_sym instead.
+	rm docs/api/symforce.cc_sym.rst
+	cp docs/symforce.cc_sym.rst docs/api/symforce.cc_sym.rst
+
+PY_EXTENSION_MODULE_PATH?=$(BUILD_DIR)/pybind
 
 docs_html: docs_apidoc
-	SYMFORCE_LOGLEVEL=WARNING $(PYTHON) -m sphinx -b html docs $(DOCS_DIR) -j4
+	export PYTHONPATH=$(PY_EXTENSION_MODULE_PATH):$(PYTHONPATH); \
+	SYMFORCE_LOGLEVEL=WARNING $(PYTHON) -m sphinx -b html docs $(DOCS_DIR) -j $$(nproc)
 
 docs: docs_clean docs_html
 

docs/symforce.cc_sym.rst

@@ -0,0 +1,11 @@
+symforce.cc\_sym module
+=======================
+
+..
+   Comment: This file exists because we want docs for cc_sym, not symforce.cc_sym.
+   This file differs only from the one generated by api-docs by the target module.
+
+.. automodule:: cc_sym
+   :members:
+   :undoc-members:
+   :show-inheritance: