Update main CTA text in landing page

[delucis]

Description (required)

This PR updates the main call-to-action link text in the docs landing page.

The current link reads “Get started”, but that is the same text as the CTA on https://astro.build/ which means users can be presented by two consecutive “Get started” choices.

The new link text is “Install Astro”, which is also a more precise description of the link destination as it points to the install guide.

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Name	Status	Preview	Comments	Updated (UTC)
docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 8, 2024 2:26pm

1 Ignored Deployment

Name	Status	Preview	Comments	Updated (UTC)
docs-i18n	⬜️ Ignored (Inspect)			May 8, 2024 2:26pm

[astrobot-houston]

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Locale	File	Note
en	getting-started.mdx	Source changed, localizations will be marked as outdated.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.