feat: update the docs for Sätteri being the default Markdown processor

[Princesseuh]

Description (required)

In V7 Sätteri is the default processor, as such some sections needs to be rewritten, upgrade guide needs to be updated, etc.

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[ArmandPhilippot]

Thanks, Erika!

I don't know how many people are using recma plugins, but I think the upgrade guide should mention it since they are not supported by Sätteri. I also think we should say Sätteri support (different kind of) plugins.

Otherwise, I left a nits on the other files but this looks good!

[matthewp]

The next time this PR updates it should trigger a new deployment which should work.

[matthewp]

@ArmandPhilippot this is ready for review again

[ArmandPhilippot]

Thanks! Glad GitHub didn't mess up with the suggestions 😅

As long as the compressHTML change (withastro/astro#16965) is not merged (we need an upgrade guide section), docs are ready for a new beta release!

Note that I already merged the related config changes (#14067); I forgot this wasn't release yet.

[matthewp]

Ok, will hold off on that one until the next beta then. Thanks!