meson: install markdown into 'docdir'

Meson doesn't have an official @docdir@ option like autotools does,
but this is typically @datadir@/doc.  Move the install of markdown
files there.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I43a9bb57c47d31bb2c6a5fe99cae8157c4694478
diff --git a/meson.build b/meson.build
index 249be71..2e9f09d 100644
--- a/meson.build
+++ b/meson.build
@@ -103,7 +103,7 @@
 install_subdir(
     generated_root,
     exclude_files: [ generated_headers, generated_cpp ],
-    install_dir: get_option('datadir') / meson.project_name(),
+    install_dir: get_option('datadir') / 'doc' / meson.project_name(),
     strip_directory: true,
 )