Reworks the security docs landing page

[benironside]

Fixes #4527 by making a first pass at a newer Elastic Security landing page, following our team's discussion of ways to improve this page. Does not yet include the advanced (box-style) formatting present in the Observability and overall docs landing pages, but is intended as a starting point for this work / step in the right direction.

We need to decide as a team whether to implement some version of this, or to wait until we can implement a more highly-formatted / CSS-reliant version. I'm leaning towards implementing some version of this since I think it provides a more streamlined UX than the existing page, and we can implement it very soon — but open to other opinions.

This PR also moves content about Endpoint self-protection features to a new page, while preserving the section header on the landing page because it's linked to from this fleet overview page. Once we merge this PR and the new self-protection page exists, I can change the link on that fleet page to go to the new page, and then in a new PR I can remove the section from the security landing page.

As you review, please consider changes we could make that would improve how well this page functions as a landing page / starting point, and whether it's an improvement over our existing page and therefore worth implementing in the short term. Thanks!

Previews: Elastic Security overview/landing page
Endpoint self-protection

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[benironside]

@elasticmachine run elasticsearch-ci/docs

[benironside]

@elasticmachine run elasticsearch-ci/docs

[nastasha-solomon]

This is a great first draft, @benironside! I left some suggestions for organizing the content on the overview page. Let me know your thoughts :)

[joepeeples]

Really great, streamlined overview! A few fairly localized suggestions to consider, thanks for taking on this effort!

[joepeeples]

These pages feel a little out of place here. They're admin-level setup and reference stuff, vs analyst-level UI features like the rest of this section (response console, trusted apps, event filters, etc.). Maybe "Get started > Endpoint requireements" or some other sub-section in "Get started"?

[benironside]

This is a good point and I'm making a note of it, but I'd like to do it in a separate PR to keep this one focused, if that's alright.

[natasha-moore-elastic]

Left a couple of suggestions but looks great overall! 🚀