docs: fix z-index across projects

[shahednasser]

These changes do not affect the main docs as there are different layers to it since it's built with Docusaurus.

• Fix z-index of different components, mainly the Navbar and Sidebar components.
• Allow scroll only in content, i.e. give a fixed height to the body and set overflow on content
• Add z-index class to Search component in docs project due to docusaurus's z-index usage.

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Latest commit: 1cb4329

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Name	Status	Preview	Comments	Updated (UTC)
api-reference	🛑 Canceled (Inspect)			Sep 27, 2023 1:21pm
docs-ui	🛑 Canceled (Inspect)			Sep 27, 2023 1:21pm
medusa-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 27, 2023 1:21pm