docs: remove references to discontinued Marqo Document Store

[veeceey]

Summary

Removes all references to the discontinued Marqo Document Store from the documentation.

Changes made:

• Removed Marqo from the list of pure vector databases in /docs-website/docs/concepts/document-store/choosing-a-document-store.mdx
• Updated the document store diagram alt text to remove Marqo
• Removed the MarqoDocumentStore integration tile from the sidebar navigation

Fixes #10487

Test plan

• Documentation builds without errors
• Sidebar navigation no longer shows MarqoDocumentStore link
• Document store page no longer references Marqo
• Diagram alt text accurately reflects available integrations

[vercel]

@veeceey is attempting to deploy a commit to the deepset Team on Vercel.

A member of the Team first needs to authorize it.

[CLAassistant]

All committers have signed the CLA.

[veeceey]

This PR is ready for merge, but is currently blocked by a Vercel team authorization requirement. This is an infrastructure/authorization issue and not related to the code quality or testing. The PR will proceed once the Vercel team permission is granted by the maintainers.

[sjrl]

Closing since completed in PR #10531