[tmux/en] Improve style & content

Style changes
  - Site format has changed back to ~80 columns, certain rows have had line breaks removed.
  - Indentation has been adjusted globally to give additional space and consistency.
  - 'Configuring Tmux' has been separated from the preceding content due to long lines causing the entire page to scroll horizontally and block text.
  - tmux.conf has been reorganized to better explain settings categorically to new users.

Content Changes
  - Unmapping the prefix key contained erroneous data