docs: modernize typography, spacing, and code block design - #6803

The typography/color/spacing work looks great — dark brand-tinted code surface with the custom Prism palette is a real improvement, and the table/alert/nav polish reads calmer than what's on main.

Two blockers before this lands, both about JS that got removed or referenced but never added:

1. MDXComponents.ts deletion breaks all <LiteYouTubeEmbed> docs. The file was the sole place where LiteYouTubeEmbed was registered as a global MDX component (via @theme-original/MDXComponents + spread + LiteYouTubeEmbed). Removing it leaves <LiteYouTubeEmbed id="…" /> in the following files with no component to resolve to, so those pages will either fail the MDX compile or render nothing:

   • docs.feldera.com/docs/literature/videos.mdx (five embeds)
   • docs.feldera.com/docs/use_cases/real_time_apps/part1.mdx
   • docs.feldera.com/docs/use_cases/batch/intro.md
   • docs.feldera.com/docs/use_cases/fine_grained_authorization/intro.md

   The react-lite-youtube-embed dep is also still in package.json, which is a good hint that the removal was accidental. Either keep the file (its only purpose was that registration), or import LiteYouTubeEmbed explicitly at the top of each .mdx / .md that uses it (note: bare .md files can't import; those would need to be renamed to .mdx).

2. .table-scroll-container CSS is dead code. The new rules in custom.css are gated on a wrapper element (.theme-doc-markdown .table-scroll-container, then .theme-doc-markdown .table-scroll-container table) that nothing in the repo produces. The inline comment says "MDXComponents wraps every markdown table in .table-scroll-container" — but (a) the pre-existing MDXComponents.ts never did that (it only added LiteYouTubeEmbed), and (b) this PR deletes it entirely. Result: tables on the rendered site fall through to the base .theme-doc-markdown table block below (which now has border: none on cells and no wrapper), so tables get quiet borderless cells but lose the rounded/bordered container the design comment describes.

   Fix is one of: (a) actually swizzle @theme/MDXComponents (or @theme/MDXContent) to wrap table elements in a <div className="table-scroll-container">, (b) drop the wrapper-dependent rules and move the border/radius/overflow onto .theme-doc-markdown table directly, or (c) drop them entirely.

Smaller notes (non-blocking):

• docusaurus.config.ts: theme and darkTheme both set to felderaCodeTheme. Intentional since the code block is dark in both modes — fine, just worth a one-line comment saying so, since the two-key API otherwise looks like a mistake.
• The Prism theme's rationale comment about property contrast is genuinely helpful — keep that style of "why this color, verified against this background" for anyone tweaking the palette later.
• SandboxButton/index.tsx: removing the stray console.log('styles', styles) is a nice drive-by, thanks.

Once (1) and (2) are addressed I'm happy to re-review — the rest of the CSS work reads solid.

Both blockers from my previous review are addressed by 1a8562ac:

• MDXComponents.tsx restores the LiteYouTubeEmbed registration.
• A table mapping wraps <table> in the .table-scroll-container div, so the new table CSS is actually applied.

The follow-up e9fb93be (consistent SQL casing across docs) is unrelated cleanup and looks fine. Approving.