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

[not-my-profile]

I'd prefer only-light-themes and only-dark-themes for the class names ... since the negation is a bit unintuitive.

There is also some precedence for that, see e.g. the mkdocs material documentation.

[mlange-42]

Thanks for the review, you are probably right here. Will try to rework this accordingly.

[2bndy5]

Technically, this fenced block is only html syntax.

Some MD linters prohibit the use of raw HTML, so maybe change this to

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

as hayesall reported it working.

[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)

[rustbot]

☔ The latest upstream changes (possibly #2681) made this pull request unmergeable. Please resolve the merge conflicts.