Correct path to README.md and include that into the API doc. (unicode-org#150)

Author: FrankYFTang

inflection/README.md

@@ -1,7 +1,7 @@
 <!--
  Copyright 2016-2024 Apple Inc. All rights reserved.
 -->
-# Inflection
+# Inflection  {#mainpage}
 
 ## About Unicode Inflection
 
@@ -125,4 +125,4 @@ cmake --build . -j8 -t check
 Optionally, ICU_ROOT can be specified in the file options.mk with the following type of syntax.
 ```
 ICU_ROOT=<PATH_TO_ICU>
-```
+```

inflection/docs/Doxyfile

@@ -943,9 +943,8 @@ WARN_LOGFILE           =
 # spaces. See also FILE_PATTERNS and EXTENSION_MAPPING
 # Note: If this tag is empty the current directory is searched.
 
-INPUT                  = ../../build/headers \
-                         ../../build/inflection_headers \
-                         ../README.md
+INPUT                  = ../../build/inflection_headers \
+                         ../../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
@@ -1142,7 +1141,7 @@ 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 = ../README.md
+USE_MDFILE_AS_MAINPAGE = ../../README.md
 
 # The Fortran standard specifies that for fixed formatted Fortran code all
 # characters from position 72 are to be considered as comment. A common