Make it light mode by default

Summary:
The new version's disableSwitch isn't equivalent to disabling dark mode like it did before. The default is still dark but now there's no way to change it.

This makes the default light again. Would be nice to enable the dark mode switch, we just need someone to make sure the CSS colours have good enough contrast etc. PRs very welcome!

Dark mode:

{F287823751}

Reviewed By: mweststrate

Differential Revision: D23161145

fbshipit-source-id: fa887afbffa8a1e1e753833b2dba618c5f93435f

website/docusaurus.config.js

@@ -35,6 +35,9 @@ const siteConfig = {
       ],
     },
     colorMode: {
+      // Nothing against dark mode, but our current CSS doesn't have high contrast
+      // so it needs some work before being enabled.
+      defaultMode: 'light',
       disableSwitch: true,
     },
     footer: {