Add support for grid gutters to control spacing between grid items

[agriyakhetarpal]

This PR closes #596. This was already supported in our sphinx-design CSS, just that we didn't expose it as a parameter in the grid shortcode, so it was partially implemented (no pun intended :P).

The spacing scale formula is defined here:

scientific-python-hugo-theme/assets/theme-css/sphinx-design/_spacing.scss

Lines 1 to 9 in b3056c3

	$spacer: 1rem;
	$spacings: (
	0: 0,
	1: $spacer * 0.25,
	2: $spacer * 0.5,
	3: $spacer,
	4: $spacer * 1.5,
	5: $spacer * 3,
	) !default;

which generates the responsive sd-g-* classes that control the --sd-gutter-x and --sd-gutter-y variables.

The gutter parameter can either take one or four integers between 0 to 5. The default is 2 for backwards compatibility. The four integers in the value, if set, correspond to the xs/sm/md/lg breakpoints. See the Bootstrap docs for more, which is what sphinx-design relies on: https://getbootstrap.com/docs/5.2/layout/grid/#grid-options

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