docs: reconfigure libbpf documentation syncing

This adds documentation files, including ones for autogenerating API
documentation based on code comments in the source code that's pulled
in via the mirror.

Signed-off-by: Grant Seltzer <grantseltzer@gmail.com>

docs/sphinx/Makefile

@@ -0,0 +1,9 @@
+SPHINXBUILD   ?= sphinx-build
+SOURCEDIR     = ../src
+BUILDDIR      = build
+
+help:
+	@$(SPHINXBUILD) -M help "$(SOURCEDIR)" "$(BUILDDIR)"
+
+%:
+	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)"