Fixed example of line spreading, color key, and cmap for antialiasing

[jbednar]

The Large Data guide had a few issues:

• It had been claiming that dynspread worked for paths and curves, but that hasn't been true since 6/2021 when we fixed dynspread to have reasonable behavior. Since then, any connected path will have no dynamic spreading, since all pixels have a connected neighbor. Suggested alternatives instead.
• The fake categorical color_key example was using [0,0] as the data, which works for some reason, but I think it needs to be ([0,0]), i.e. a single point (x,y)
• The new examples of line_width=X weren't showing antialiasing, because they had only a single color in their colormap. Added colormaps for those.
• The seemingly valid syntax (c + m).opts(opts.RGB(width=400)) has started showing WARNING:param.main: Calling the .opts method with options broken down by options group (i.e. separate plot, style and norm groups) is deprecated., which seems like a bogus warning since the option is applied and is not broken down by options group. Silenced it by switching to c.opts(width=400) + m.opts(width=400), but that code was previously legal and presumably should not be warning.

[codecov-commenter]

Codecov Report

Merging #5267 (c69d7ee) into master (7930d08) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5267   +/-   ##
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  Coverage   86.72%   86.72%           
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  Files         299      299           
  Lines       62187    62187           
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  Hits        53929    53929           
  Misses       8258     8258           

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