Update mdx integration docs for v3

[bluwy]

Description (required)

Update the docs for the MDX integration's next major (v3). The main change is the rename of optimize.customComponentNames -> optimize.ignoreElementNames.

There's some other technically breaking changes listed in the linked PR below, but I don't think it will affect most users, so I didn't add a new migration section.

Related issues & labels (optional)

For Astro version: 4.8. See astro PR withastro/astro#10935

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Name	Status	Preview	Updated (UTC)
docs	✅ Ready (Inspect)	Visit Preview	May 8, 2024 3:42pm

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[astrobot-houston]

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en	guides/integrations-guide/mdx.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[sarah11918]

This looks great @bluwy ! I just used our <Since> component to call attention to the change with the new version, which will need to be imported at the top of the file, (which I can't just suggest here in GitHub):

import Since from '~/components/Since.astro';

[sarah11918]

Looks great! Ready for 4.8 💪

[sarah11918]

Updating branch and pointing to new 4.8.0 branch