feat(root): add colour tokens for table of contents light and dark mode

[GCHQ-Developer-847]

Summary of the changes

Added a color-mode.css file to the website. Added colour tokens for light and dark mode of the table of contents / anchor nav component.

I have only added theme changes / code for the table of contents - I believe the rest (e.g. wrapping the whole website in an ic-theme component? Etc.) will be done in #1133.

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To view the changes in this PR, add the following to the styles in Chrome DevTools:

:root {
  color-scheme: light dark;
}

and then click on the paintbrush icon to change between modes.

Related issue

Part of mi6/ic-ui-kit#1214

Checklist

• [x] I have manually accessibility tested any changes, if relevant.

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