AutoEmbed.rb is a plugin for Jekyll that automatically finds and renders embeds for any "naked URL" on the site.

AutoEmbed.rb

# AutoEmbed plugin for Jekyll
#
# Created by Eric Celeste for Tenseg LLC
# March 2019
#
# License: MIT
#
# This plugin uses the ruby-oembed library to look up an oembed for
# any "naked URL" (a URL in a paragraph by itself). If the oembed
# is found, then the HTML for the oembed is presented. If there is
# no oembed found, then the URL is simply turned into a link.
#
# For best results, this plugin should be combined with some SCSS
# to improve the visual presentation of the oembeds. This includes
# ensuring that videos are presented with their proper aspect ratio
# and that some links that clutter rich embed results are hidden.
#
# This is the recommended SCSS to accompany this plugin:
#
# .embed-container {
#     width: 100%;
#     iframe {
#         width: 100%;
#     }
#     &.rich {
#         div, blockquote, p {
#             display: none;
#         }
#     }
#     &.video {
#         display: block;
#         position: relative;
#         width: 100%;
#         height: 0;
#         padding-bottom: 51%;
#         iframe {
#             display: block;
#             position: absolute;
#             width: 100%;
#             height: 100%;
#             top: 0;
#             left: 0;
#         }
#     }
# }

# see https://github.com/ruby-oembed/ruby-oembed
require 'oembed'

# register all default oembed providers
::OEmbed::Providers.register_all
# add providers and fallbacks here, if you like

Jekyll::Hooks.register [:pages, :documents], :post_render do |doc|
    if (doc.is_a?(Jekyll::Page) || doc.write?) && (doc.output_ext == ".html" || (doc.permalink&.end_with?("/")))
        # look for naked URLs wrapped in paragraph tags
        doc.output = doc.output.gsub(/<p>(https?:\/\/[\S]+)<\/p>/) do |match|
            out = '' # use this variable so we can include debugging info if we wish
            begin
                # if an oembed is found, then include the embed wrapped in a div
                result = ::OEmbed::Providers.get($1)
                fields = {
                    'provider_name' => result.fields['provider_name'],
                    'title' => result.fields['title'],
                    'type' => result.fields['type'],
                    'width' => result.fields['width'],
                    'height' => result.fields['height'],
                }
                style = ''
                if fields['type'] == 'video'
                    fields['ratio'] = fields['height'].to_f / fields['width'].to_f * 100
                    style = "padding-bottom: #{fields['ratio']}%"
                end
                # out += "<pre>result: #{JSON.pretty_generate(fields)}</pre>"
                out += "<div class=\"autoembed embed-container #{result.fields['type']}\" style=\"#{style}\">#{result.html}</div>"
            rescue StandardError => e
                # if no oembed is found, just turn the naked URL into a link
                # out += "<pre>#{e.message} #{e.backtrace.inspect}</pre>"
                out += "<p class=\"autoembed no-embed\"><a href='#{$1}'>#{$1}</a></p>"
            end
            out
        end
    end
end

To install this plugin on a Jekyll site, just put the AutoEmbed.rb file in your _plugins directory. You should also add ruby-oembed to your gems and the recommended styles to your SCSS.