Add basics programs and distribution.

Use groups to locally modify the description label.
Change description list into tabular.

+ Add LaTeX package 'tabularx' to allow for width to stretch to
  textwidth.

+ Add further entries.

+ Highlight letters that give the program name from description.
  Set highlight style to bold.

+ Add builtin column.
Add forgotten programs.

rm, rmdir and mkdir are essential programs for new users.
Add list command explanation and common options.
Add cd and pwd explanation and example.
Change to GNU manual descriptions.

Mac OS X (BSD) manual descriptions and 'builtin' were used.
Detail rm, cp and mv programs.
Add description of mkdir and rmdir.
Fix typo in command.

+ Extra a/b/c in mkdir command.

+ paragraph align a section.
Add description for cat and less.
Add description of echo and clear.
Merge branch 'feature/basic' into feature/terminal

Merge in the basic program and commands subsections into the terminal
section.