Writing custom check functions vignette

[annakrystalli]

This PR adds:

• A new vignette on how to create custom validation checks for hub validations (Resolves Document requirements for custom validation functions #121)
• new sections on how to manage additional dependencies required by custom validation functions (Resolves Develop mechanism for managing custom function additional dependencies #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]

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

[LucieContamin]

Thank you very much! It globally looks all good and make sense. I like the examples. I just made some minor comments.

[zkamvar]

This is a monumental effort! Thank you for writing this, Anna!

One minor comment about formatting: Vignettes should start with heading level 2. The title takes the level 1 heading and pkgdown will automatically convert all the headings for you. This is an accessibility technique to create semantic structure: https://webaim.org/techniques/headings/

Otherwise, I have some suggestions and clarifications that I think will strengthen the vignette. I felt that the discussion in _custom-fn-available_args was a bit difficult to understand and I attempted to have a go at it (though I don't know how well I did).

[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!

[zkamvar]

This looks great! Thank you for your diligence on this.

I have a couple of minor formatting changes that ensures the alert shows up, ensures the lists are formatted correctly, and that headings are headings.

[LucieContamin]

It looks all good to me! thank you very much for all the work!

I just made some small suggestions, mostly about stuff not rendering properly on my side. If it's just on me, please ignore!

[annakrystalli]

Thanks again both!

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

[zkamvar]

It looks good! The only thing left is to make sure the subheadings are actually marked as headings or combined with their parents.

I'm not trying to bike shed, I'm thinking about this from an accessibility standpoint. At the moment, they are only sub headings for people who use vision to read the document. For people who use screen readers, they are sentence fragments.

Here are a couple of resources that describe this:
WebAIM: Non-HTML headings (visual only)
WCAG 2.2: Failure of Success Criterion 1.3.1 due to using changes in text presentation to convey information without using the appropriate markup or text (note that the WCAG is the authority for internet accessibility).

If you don't think the subheadings add much to the structure, then I would suggest to combine them with the headings:

### Capturing and returning check results with `capture_check_cnd()`

### Skipping checks and returning a message with `capture_check_info()`

[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