Apply line breaks between headings automatically

**Is your feature request related to a problem? Please describe.**
Line breaks only appear automatically under h2's, but ideally we'd want to set defaults for all headers to have them above and below, as having them set manually around this in the .qmd files is chaos - As Cam suggested in [this issue](https://github.com/dfe-analytical-services/analysts-guide/issues/70)

**Describe the solution you'd like**

- [ ] Put this code in the _styles.css file:

`// Apply line breaks between headings
h1, h2, h3, h4, h5, h6 {
position: relative;
&:before,
&:after {
content: "";
display: block;
width: 100%;
height: 1px;
margin: 0.5em 0;
background-color: #adafae;
}
}`

- [ ] Delete manual line breaks from qmd files