Improve formatting of configuration directives in the documentation

[cuu508]

Maddy's configuration directive docs are incredibly useful, but are not easy to scan visually. The reader must find the right line that starts with "Syntax:", then read the section, without "overshooting" into the next section:

I'm proposing a few formatting changes to make each section easier to find visually:

I moved the "Syntax: " line to a <h3> element, and added <hr> separators between sections. I removed bold from the "Default:" line to reduce visual clutter. I made a tiny CSS change in the alb theme to make <hr> element thinner.

Markdown source (fragment):

### debug _boolean_

Default: global directive value.

Enable verbose logging.

---

### domains _string list_

**Required**. <br>
Default: not specified.


ADministrative Management Domains (ADMDs) taking responsibility for messages.

Should be specified either as a directive or as an argument.

---

@foxcpp what are your thoughts on this? Yay or nay? I'd be happy to make a PR (or several smaller PRs) that reformat the docs.

[foxcpp]

That looks like a good improvement! PRs are welcome.

[foxcpp]

Sources for the theme are here: https://github.com/foxcpp/mkdocs_alb