auto-gen docs of flex (#2851)

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## What do these changes do?

<!-- Please give a short brief about these changes. -->

Auto-gen the docs of `flex` and remove the pre-built html files. Due to
the reduction of dependencies at build time (e.g. using cmake for flex),
a new Doxygen file was created, namely `Doxygen-flex`.

## Related issue number
#2726

Signed-off-by: Sijie Shen <shensijie.ssj@alibaba-inc.com>

docs/Doxyfile-flex

@@ -0,0 +1,21 @@
+PROJECT_NAME = "Flex"
+PROJECT_NUMBER = "0.17.9"
+OUTPUT_DIRECTORY = "_build/latest/html/reference/"
+HTML_OUTPUT = "flex"
+
+INPUT = ../flex/storages/rt_mutable_graph \
+        ../flex/engines/graph_db/app \
+        ../flex/engines/graph_db/database \
+        ../flex/utils \
+
+FILE_PATTERNS = *.h *.cc
+RECURSIVE = YES
+OPTIMIZE_OUTPUT_FOR_C = NO
+
+EXTRACT_ALL = YES
+EXTRACT_PRIVATE = YES
+EXTRACT_STATIC = YES
+
+TYPED_HIDES_STRUCT = YES
+GENERATE_LATEX = NO
+GENERATE_HTML = YES

docs/Makefile

@@ -41,6 +41,9 @@ clean:
 	cd ${WORKING_DIR}/../analytical_engine/java && \
 	mvn -P javadoc javadoc:aggregate -Dmaven.antrun.skip=true -DskipTests -Djavadoc.output.directory=${WORKING_DIR}/$(BUILDDIR)/${TAG}/$@/reference -Djavadoc.output.destDir=gae_java --quiet \
 
+	@echo "Building Java doc to $(BUILDDIR)/${TAG}/$@/reference/flex"
+	@doxygen Doxyfile-flex
+
 doxygen:
 	@mkdir -p _build
 	@doxygen Doxyfile

docs/index.rst

@@ -117,6 +117,7 @@ and the vineyard store that offers efficient in-memory data transfers.
    reference/python_index
    reference/analytical_engine_index
    Analytical Engine Java API Reference <reference/gae_java/index>
+   Flex API Reference <reference/flex/index>
 
 .. toctree::
    :maxdepth: 2
@@ -130,4 +131,3 @@ Indices and tables
 * :ref:`genindex`
 * :ref:`modindex`
 * :ref:`search`
-

docs/reference/flex/index.rst

@@ -0,0 +1,4 @@
+Flex API Reference
+========================
+
+Stub page for the Flex API reference docs; actual source is located in the flex/ directory.