Add CSS styles for fake header rubrics

[tetrapod00]

Followup to #9989.

The sphinx rubric directive is used in place of real semantic headers in some index pages. This change adds CSS styles that mimic the current real h2 and h3 styles, and applies those to the fake header rubrics.

Only fake-h2 is used in this PR, but fake-h3 will be needed for the changes in #10110.

I followed this stackoverflow answer to implement this.

Current h2 and h3 styles are pulled from https://github.com/readthedocs/sphinx_rtd_theme/blob/7b894f5e7a5053dd35de406bc779020c229f20a9/sphinx_rtd_theme/static/css/theme.css and from:

godot-docs/_static/css/custom.css

Lines 304 to 314 in 9a44730

	h1,
	h2,
	h3,
	h4,
	h5,
	h6,
	legend,
	.rst-content .toctree-wrapper p.caption {
	/* Use the same font as the godotengine.org website. */
	font-family: var(--header-font-family);
	}

Comparison of Building from Source

Master (after #9989). Note the tiny rubric headers:

This PR. Note that headers match the existing h2 style:

4.2 (Before #9989):

[mhilbrunner]

Note to self everyone as discussed on RC: We'll postpone this one post the impending Sphinx update, and see if we can make do without (maybe with this)

[tetrapod00]

With Sphinx updated, I tried out the alternative using the :heading-level: option for the .. rubric:: directive and it seems to work perfectly. So this PR won't need to add custom CSS.

[mhilbrunner]

Superseded by #10187.