doc: various small style improvements

- settle on a single &R; entity, replacing &r;
- get rid of &kapp; in favour of &rkward;
- use entities more: &rkward;, &GUI;, &XML;, &HTML;, &ie;, ⪚,
  &javascript;, &URL;, etc
- remove contractions ("don't" -> "do not", etc)
- switch URLs within kde.org to https
- remove extra exclamation marks
- remove few trailing whitespaces
- few more minor fixes