Allow disabling light/dark mode toggle

[Eric-Arellano]

Our project is excited to add dark mode support thanks to Furo. It's a big reason we switched themes!

But, we need to do some work first like choosing our color scheme & auditing all projects' images. We don't want to block switching to Furo on this.

So, we'd love the option in conf.py to disable the theme toggle button:

furo/src/furo/theme/furo/page.html

Lines 36 to 43 in e7f732e

	<div class="theme-toggle-container theme-toggle-header">
	<button class="theme-toggle">
	<div class="visually-hidden">Toggle Light / Dark / Auto color theme</div>
	<svg class="theme-icon-when-auto"><use href="#svg-sun-half"></use></svg>
	<svg class="theme-icon-when-dark"><use href="#svg-moon"></use></svg>
	<svg class="theme-icon-when-light"><use href="#svg-sun"></use></svg>
	</button>
	</div>

furo/src/furo/theme/furo/page.html

Lines 76 to 84 in e7f732e

	{#- Theme toggle -#}
	<div class="theme-toggle-container theme-toggle-content">
	<button class="theme-toggle">
	<div class="visually-hidden">Toggle Light / Dark / Auto color theme</div>
	<svg class="theme-icon-when-auto"><use href="#svg-sun-half"></use></svg>
	<svg class="theme-icon-when-dark"><use href="#svg-moon"></use></svg>
	<svg class="theme-icon-when-light"><use href="#svg-sun"></use></svg>
	</button>
	</div>

I'd be happy to add this if you'd like :)

[pradyunsg]

See #28