Standardise appearance of variable and other styles in articles

[delphidabbler]

The appearance of variable values in articles is all over the place.

Create a new CSS class, say .value, that is used across the board to style values assigned or expected from variables or property values etc.

Also standardise use of <samp> for sample or expected output and <kbd> for what needs to be typed on the keyboard, unless it's a single key press when .keycap should be used.

Also consider using new class .filename for styling file names instead of using <code> as now. As an aside, make sure that unit names sans extension are styled with <var> instead of <code>.

[delphidabbler]

Also need to consider creating a CSS class, say .compdir for compiler directives. Currently <code> gets used for this.

[delphidabbler]

Some HTML tags are also rendered as <code> - these are now to be rendered using the .kwd class, as some have.

.kwd is now being used when referring to Pascal keywords in text. This too needs standardising.