Writing custom check functions vignette

[annakrystalli]

This PR adds:

• A new vignette on how to create custom validation checks for hub validations (Resolves #121)
• new sections on how to manage additional dependencies required by custom validation functions (Resolves #22).

I've also pulled out information that is useful to both the writing and deploying custom functions vignette into children documents which are now deployed in both. Because of this it might be more useful to have the rendered vignette open to see the content in place.

[github-actions[bot]]

🚀 Deployed on https://6709467a594ae5042e3e833d--hubvalidations-pr-previews.netlify.app

[annakrystalli]

Thanks both @LucieContamin & @zkamvar ! I've taken your super helpful suggestions and reworked the vignettes so hopefully things are clearer. Let me know your thoughts!

[annakrystalli]

Thanks again both!

I incorporated the majority of your suggestions. Hopefully we're good to go now 😅

[annakrystalli]

OK. I don't love such long headers and feel it visually burdens the page now but I've added them anyways to move this forward.

I did also remove the level 3 heading introduced in the warning box about tbl. Marking info as headers when they are not (it's really a box title) is also not good for accessibility. Most importantly it had no business in the TOC