docs: add <StarlightPage> import order guidelines

[HiDeoo]

Description

This PR updates the <StarlightPage> component documentation to help with issue 2 in #3162:

After trying many possible workarounds to find a way around what seems to be an unsolvable issue, which depends on how import order works in Astro and also many variables e.g. the number of routes, etc., this PR adds guidelines to the documentation regarding the import order of the <StarlightPage> component and its role in setting up cascade layers.

This first attempt is based on recommendations from Sarah during a Talking & Doc'ing session mentioning the limitations using text and code example comments.

I tried to avoid an aside even tho we use one in the Astro Docs:

Considering how tricky this is, maybe it should be an aside but I wanted to try a first approach without it. Open to any suggestions.

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[alvinometric]

Not sure if this needs a review by @sarah11918 ?

[delucis]

Seems fair to document this workaround for now. Hopefully we can fix this properly soon!

[louisescher]

Such PR, much wow!