How to automatically add Read more section after the first paragraphe in post?

Use the excerpt with a custom read more link in functions.php

add_filter( 'excerpt_length', 'modify_excerpt_length' );

function modify_excerpt_length( $length ) {

    return 15; 
}


add_filter( 'excerpt_more', 'your_excerpt_more' );

function your_excerpt_more( $more ) {

$more = sprintf( ' <a href="https://wordpress.stackexchange.com/questions/341980/%s" class="read-more" rel="bookmark">' . __( 'Read More' ) . '</a>', esc_url( get_permalink() ) );

return $more;
}

Or, you can filter the_excerpt or get_the_excerpt hook like this :

add_filter( 'get_the_excerpt', 'link_read_more' );

function link_read_more( $output ) {

$more = sprintf( ' <a href="https://wordpress.stackexchange.com/questions/341980/%s" class="read-more" rel="bookmark">' . __( 'Read More' ) . '</a>', esc_url( get_permalink() ) );

return $output . $more;
}