Generate Client API Doxygen documentation in OSS docs target (#44)

Summary:
Put the config files used for generating the LogDevice Client API doxygen
documentation under source control. The following, limited amendments
have been made:

* Files renamed and re-located so that in code paths are relative to top
of LogDevice source tree
* Doxyfile standard downgraded from 1.8.14 to 1.8.13; as delivered with
Ubuntu Bionic, the primary supported distrib. (i.e. doxygen -ui filename)
* Corrected typo in footer as per issue #36
Pull Request resolved: #44

Differential Revision: D10297560

Pulled By: calebmarchent

fbshipit-source-id: c22262754c057ce6b648aed5bf73359447c77a46

logdevice/CMake/build-docs.cmake

@@ -8,5 +8,9 @@ add_custom_target(ldquery.md
 	WORKING_DIRECTORY ${LOGDEVICE_DIR}/../docs
 	DEPENDS markdown-ldquery)
 
+add_custom_target(client-api-doxy
+	COMMAND rm -rf website/static/api && doxygen logdevice/build_tools/Doxyfile.client-api
+	WORKING_DIRECTORY ${LOGDEVICE_DIR}/..)
+
 add_custom_target(docs)
-add_dependencies(docs settings.md ldquery.md)
+add_dependencies(docs settings.md ldquery.md client-api-doxy)