doc: rewrite font sizes, add viewport meta

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

doc

Rewrote pretty much all font size definitions in the docs CSS. The viewport meta lets mobile browsers know the site is mobile-ready, which made most of the font size hacks obsolete.

I converted most em units to rem because cascaded font sizes can lead to unwanted surprises. I retained a 90% size on monospace fonts because they appear to be more in-line with the variable width font. I also resolved a few issues that I've noticed surrounding the version selection dropdown, and one issue of a heading layout overflow.

Screenshots at various widths:

Obsoletes: https://github.com/nodejs/node/pull/15436