Document how to adjust the font size

[Nerixyz]

Basically the same change as in #8.

I was trying to adjust the font, which you can, for #raw, do by using #show raw: it => ..., but here you'll need to use raw.line.
To save the next person some work, I've added an example to the manual.

I've marked this as a draft, as it depends on #13 (I'll rebase once it's merged).

[Galfy1]

maybe also add something like:
#show figure.where(kind: raw): set par(justify: false)
(justification will mess up indentation if a code line is too long and wraps)

[Galfy1]

and something like "#show figure.caption: set text(size: default_font_size)". to make sure the caption size does not get smaller, just because we set a smaller font size for figure.where(kind: raw)