Add "read more" link to excerpts generated using the "more tag"

[jamiebergen]

• I changed the_content() to the_excerpt() in template-parts/content.php if the post is displayed on an archive page so that excerpts are automatically generated.
• However, that means that posts using the "more tag" won't get the excerpt more link that automatically generated or hand-crafted excerpts get (see inc/template-functions.php)
• See the post titled "Template: More Tag" in the WordPress theme as an example.