Improve text sizes in Markdown rendering

[Fs00]

I've made a couple of adjustments regarding text sizes to Markdown rendering in comments/Readme:

• _subscript/^superscript text is now smaller, to make it look as it does in GitHub web UI
• sizes of H1 to H6 has been changed to make the different types of heading more clearly distinguishable. Previously I had a hard time finding the difference between an H1 and an H2, or between an H2 and an H3. You can see below the difference:

Before	After

[Fs00]

@maniac103 Can this be merged too? Any doubts?

[maniac103]

The only thing I have doubts about is that I'm not sure about H6 size being smaller than normal text.

[Fs00]

I thought it was fine because I've seen that the GitHub UI makes H6 even smaller than we do (on the GH website H6 are 85% of the normal text size compared to 90% in this PR).

An alternative I see is to make H6 as big as normal text and H5, but change their color to a dark shade of grey (both the GitHub UI and our Markdown CSS use this approach).

[maniac103]

If they're doing the same thing it's fine I guess.