Fix doxygen generation (#103)

* Fix doxygen generation

* Fix outdated MainPage

* Fix outdated protobuf docs

Author: BewareMyPower

.gitignore

@@ -72,6 +72,7 @@ compile_commands.json
 
 # doxygen files
 apidocs/
+doxygen/
 
 # generated files
 generated/

Doxyfile

@@ -77,7 +77,7 @@ PROJECT_LOGO           =
 # entered, it will be relative to the location where doxygen was started. If
 # left blank the current directory will be used.
 
-OUTPUT_DIRECTORY       = ../target/doxygen
+OUTPUT_DIRECTORY       = ./doxygen
 
 # If the CREATE_SUBDIRS tag is set to YES then doxygen will create 4096 sub-
 # directories (in 2 levels) under the output directory of each output format and
@@ -809,7 +809,7 @@ WARN_LOGFILE           =
 # spaces. See also FILE_PATTERNS and EXTENSION_MAPPING
 # Note: If this tag is empty the current directory is searched.
 
-INPUT                  = include docs/MainPage.md
+INPUT                  = include ./README.md
 
 # This tag can be used to specify the character encoding of the source files
 # that doxygen parses. Internally doxygen uses the UTF-8 encoding. Doxygen uses
@@ -1001,7 +1001,7 @@ FILTER_SOURCE_PATTERNS =
 # (index.html). This can be useful if you have a project on for instance GitHub
 # and want to reuse the introduction page also for the doxygen output.
 
-USE_MDFILE_AS_MAINPAGE = docs/MainPage.md
+USE_MDFILE_AS_MAINPAGE = ./README.md
 
 #---------------------------------------------------------------------------
 # Configuration options related to source browsing

README.md

@@ -21,7 +21,11 @@
 
 # Pulsar C++ client library
 
-Examples for using the API to publish and consume messages can be found under the [examples](examples) folder.
+Examples for using the API to publish and consume messages can be found under the [examples](https://github.com/apache/pulsar-client-cpp/tree/main/examples) folder.
+
+## Generate the API documents
+
+Pulsar C++ client uses [doxygen](https://www.doxygen.nl) to build API documents. After installing `doxygen`, you only need to run `doxygen` to generate the API documents whose main page is under the `doxygen/html/index.html` path.
 
 ## Requirements
 
@@ -32,8 +36,6 @@ Examples for using the API to publish and consume messages can be found under th
 * [libcurl](https://curl.se/libcurl/)
 * [openssl](https://github.com/openssl/openssl)
 
-It's recommended to use Protocol Buffer 2.6 because it's verified by CI, but 3.x also works.
-
 The default supported [compression types](include/pulsar/CompressionType.h) are:
 
 * `CompressionNone`