Enable fatal warnings in HotDoc when building website.

docs/meson.build

diff --git a/docs/meson.build b/docs/meson.build
index 7369335..82d0382 100644
--- a/docs/meson.build
+++ b/docs/meson.build
@@ -30,6 +30,7 @@
     git_upload_repository: 'git@github.com:mesonbuild/mesonbuild.github.io.git',
     edit_on_github_repository: 'https://github.com/mesonbuild/meson',
     syntax_highlighting_activate: true,
+    fatal_warnings: true,
 )
 
 run_target('upload',