[doc] Fix missing input tag at Doxygen config
`docs/checker_docs` is moved under `docs/web/checker_docs` but
at the INPUT tag values we already listed the parent directory
(`docs`) of this directory which is enough, so we remove this
directory at all.
@@ -654,7 +654,7 @@ WARN_LOGFILE =
# directories like "/usr/src/myproject". Separate the files or directories
# with spaces.
-INPUT = web/api docs docs/checker_docs
+INPUT = web/api docs
# This tag can be used to specify the character encoding of the source files
# that doxygen parses. Internally doxygen uses the UTF-8 encoding, which is
