Fix: docs should soft fail when vf-design-tokens aren't present

[khawkins98]

This is a workaround so the doc compilation in fractal will softfail when vf-design-tokens aren't present.

A few notes:

1. When the docs don't compile, the fractal components aren't available
2. This is mostly an issue for vf-eleventy sites, but fractal should be compiling the vf-core docs anyways 🤷‍♂
3. ☝️ That could also be an issue in vf-core if/when we move vf-design-tokens as an npm install

To me all that continues to suggest that we still need a more "holistic" way to approach the docs, so this PR is more of an "alpha-level" fix.

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[sturobson]

It's bizarre we're hitting this - I wonder if there's a better way. I really want to tie up the design-system site this week or next - so I'll be taking another look.

[khawkins98]

Yeah, wonder if this sort of docs should just live at the design system level?

[sturobson]

the reason for the separation at the moment is that design system is really only 'high level' so people looking at it shouldn't really care what the CSS custom property is.

Although, we could re-jig bigzing! So that the design system is actually where everything lives.

Which gets my vote :)

[khawkins98]

It kinda feels like things are tilting that way-ish. Would be a more natural fit if, sya, EMBL and EMBL-EBI "VFs" have slightly different colour palettes.