Replace Use Cases card with Quickstart

[btang05]

Deprecate link to old Use Cases page, so we stop getting spammed with requests.

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Important

Replace "Use Cases" card with "Quickstart" card in overview.mdx, updating icon and link.

• Documentation Update:
  • In overview.mdx, replace "Use Cases" card with "Quickstart" card.
  • Update card icon to fa-solid fa-bolt.
  • Change card link to https://credalai.notion.site/Credal-Quickstart-Guide-9f1d589bc51a461e9c9860c2180abc58.

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