Documentation cleanup (inc. use of <input ... readonly> for static form controls example)

following up from all the html linting errors i was getting with the new build system (see https://github.com/twbs/bootstrap/pull/22477#issuecomment-296172499)

• fixes broken code indentation (and that was a joy to do...urgh)
• remove incorrect <label>, change the static text approach and explainer to encourage use of readonly inputs instead of meaningless paragraphs
• removes mention of cursor:not-allowed which was removed in https://github.com/twbs/bootstrap/pull/22403
• removes unnecessary vendor prefixes for box-sizing code sample (which also fixes the indentation linting error)