Fix red X thumbnail appearing in Google search results

[svekars]

• Google was picking up the pytorch-x.svg image (red X close button on the cookie banner) as the page thumbnail in search results, since it was
  served as an tag — one of the few crawlable images on most doc pages
• Replaced the tag in the cookie banner with a CSS-only using ::before/::after pseudo-elements to draw the X, so there's no image for
  Google to index
• Added data-nosnippet attribute to the cookie banner wrapper as an extra signal to Google to exclude it from search snippets

Test plan

• Verify the cookie banner close button still renders as a red X and is clickable
• Verify the close button styling is correct across breakpoints (mobile, tablet, desktop)
• Inspect the page source to confirm no tag referencing pytorch-x.svg remains
• After reindexing, confirm Google search results no longer show the red X thumbnail

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