add option to specify additional HTMLs in docs

[jbloom]

You can now specify an add_htmls_to_docs variable in your Snakefile that specifies additional files to include in the docs, like this:

# add extra HTML to docs
add_htmls_to_docs = {
    "Additional files": {
        "Example HTML file": "results/extra_htmls/example_html.html",
    },
}

Also, snakemake was upgraded to 8.5.4.