Improve dev docs styling

[J5lx]

I gave the dev docs a little makeover to bring it more in line with the rest of the site, prompted by this little conversation on Discord:

scribble: I think reviewing the Contributing page is also in order. […] me: I’ve thought about that as well. I’ve also been wondering whether it might be a good idea to merge it into the https://dev.pencil2d.org/ front page and then use that as the new contribute page, so that there’s a single place for all the contribution docs. scribble: Seems like a good idea. I just wish our docs didn't look so ugly. Not exactly putting our best foot forward for potential new contributors.

Although the diff is a bit on the larger side, a fair portion of it is simply boilerplate to nullify the default styles.

Before:

After:

[J5lx]

I think I’ll simply merge this. There’s no changes to Pencil2D itself here and I’m under the impression that there’s generally not much interest in this aspect of the setup, so I don’t want to take attention away from other PRs.

[scribblemaniac]

Already looks much more cohesive now. Thanks!