Add external links class option to Markdown configuration

[welpo]

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Discussed in https://zola.discourse.group/t/style-internal-and-external-links-differently/1501/6.

This PR adds an optional external_links_class configuration option that allows adding a CSS class to all external links.

This makes it easier to style external links differently from internal ones, for example to add visual indicators for external links.

Example configuration:

[markdown]
external_links_class = "external"

with CSS:

a.external::after {
    display: inline-block;
    vertical-align: middle;
    mask-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 24 24'%3E%3Cg fill='none' stroke='currentColor' stroke-linecap='round' stroke-linejoin='round' stroke-width='2'%3E%3Cpath d='M11 5h-6v14h14v-6'/%3E%3Cpath d='M13 11l7 -7'/%3E%3Cpath d='M21 3h-6M21 3v6'/%3E%3C/g%3E%3C/svg%3E");
    margin-left: 0.1em;
    background-color: currentColor;
    width: 1em;
    height: 1em;
    content: "";
}

It looks like:

Note: for the docs, I wasn't sure whether to do (same as author):

external_links_class =

or (make it super obvious it's a string):

external_links_class = ""

Opted for author format, for consistency.

Happy to make any necessary changes!