Support a logo for the dark theme

[choldgraf]

Context

In #673 we are adding classes to support theme-specific images, content, etc. Basically, the classes will make an image display: none when the opposing theme is active.

A common use-case for theme-specific content is images, and this is one case where it'd be trickier for users to use this functionality.

Proposal

We should make it possible for users to define a "Dark Theme logo" that would be added to the site HTML with the dark-only class, and make our current html_logo image use the light-only class.

I think that the place to implement this would be in our navbar logo component:

pydata-sphinx-theme/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/components/navbar-logo.html

Lines 1 to 19 in b806673

	{% if logo %}
	{% if not theme_logo_link %}
	<a class="navbar-brand" href="{{ pathto(master_doc) }}">
	<img src="{{ pathto('_static/' + logo, 1) }}" class="logo" alt="logo">
	</a>
	{% elif theme_logo_link[:4] == 'http' %}
	<a class="navbar-brand" href="{{ theme_logo_link }}">
	<img src="{{ pathto('_static/' + logo, 1) }}" class="logo" alt="logo">
	</a>
	{% else %}
	<a class="navbar-brand" href="{{ pathto(theme_logo_link) }}">
	<img src="{{ pathto('_static/' + logo, 1) }}" class="logo" alt="logo">
	</a>
	{% endif %}
	{% else %}
	<a class="navbar-brand" href="{{ pathto(master_doc) }}">
	<p class="title">{{ project }}</p>
	</a>
	{% endif %}

And maybe the simplest config would be to also support an html_logo_dark configuration instead of just html_logo.