feat: add resizing between the docs and the playground, small HTML restructure

[ELHart05]

I have added:

• Resizing using Splitter, I discovered the tool weeks ago and I was a bit struggling with the resizing,
• HTML restructure, so we can have a (it doesn't effect the work)
• Added few styles: pointer cursors on the summary, I added the margin on y axis and padding 0 to the body so I can attribute the 100vh to the splitter.

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Name	Status	Preview	Updated (UTC)
mermaid-to-excalidraw	✅ Ready (Inspect)	Visit Preview	Apr 30, 2025 11:57pm

[ELHart05]

@ad1992 can you check this PR?

[dwelle]

Hi @ELHart05, apologies for the delay! Looks good, so we're good to merge, thanks!