Add new vignette about using manifest + board_url()

[juliasilge]

As discussed in #681, let's add a vignette on how to use a manifest with board_url(). Thank you so much for being willing to work on it @ijlyttle!

It is not a good fit for an example, because you need something like a server or webfakes to serve the directory with a manifest. Since there will be no example, let's be sure to link to the vignette in the docs for both the manifest and board_url().

[juliasilge]

I think what would make the most sense is to put a webfakes bit serving the folder in an echo = FALSE chunk and say in words, "and then you serve the board".

[juliasilge]

Let's mention that you might want to do this on GitHub. Related to #600.

[juliasilge]

I believe we are good to go on this now!

[github-actions]

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