Theme-dependent image variants

[mlange-42]

Implements theme-dependent image variants using CSS and HTML classes. Basically, HTML elements can be excluded on individual themes, or on themes grouped by light vs. dark.

See the added documentation for usage details. They also show the feature in action.

An alternative would be to let users do it via CSS themselves, but this solution might not be obvious.

Fixes #1990

[hayesall]

@mlange-42 I've been working with this for a little while now, and it seems like a good approach after not-my-profile's review:

.light img[src$="#only-dark"],
.rust  img[src$="#only-dark"] {
    display:none;
}

.navy img[src$="#only-light"],
.ayu  img[src$="#only-light"],
.coal img[src$="#only-light"] {
    display:none;
}

plus it works directly in Markdown:

![rust logo standard edition](images/rust-logo-blk.svg#only-light)
![rust logo dark mode](images/rust-logo-blk-dark.svg#only-dark)