ARM style change: render literals in black color

ARM style change: render literals in black color

After enormous amount of bikesheding about colors we decided to override
ReadTheDocs default style for literals (``literal`` in the RST markup).

Justification:
- The default RTD "light red literal on white background" is hard to
  read.  https://webaim.org/resources/contrastchecker/ reports that text
  colored as rgb(231, 76, 60) on white background has insufficient
  contrast.
- The ARM has enormous amount of literals all over the place and thus
  one sentence can contain several black/red/black color changes. This
  is distracting. As a consequence, the ARM looks like a Geronimo
  Stilton book.

What we experimented with as replacements for red:
- Green - way too distracting
- Blue - too similar to "usual clickable link"
- Violet - too Geronimo Stilton style
- Brown - better but still distracting

After all the bikesheding we settled on black, i.e. the same as all
"normal" text. I.e. the color is now the same and literals are denoted
by monospaced font and a box around the literal. This has best contrast
and is way less distracting than it used to be.

This lead to a new problem: Internal references to "term definitions"
defined using directives like .. option:: were rendered almost the same
as literals:
- References: monospaced + box + bold + clickable
- Literals: monospaced + box To distinguish these two we added black
  dotted underline to clickable references.

I hereby declare the bikeshed painted.

(cherry picked from commit 833af31e7b83784e1bdfb49a43974810a9ee3dd0)