[tmux/en] Structural updates & additional content

+ Fixed some spelling, grammar, and punctuation issues
+ Separated terminal commands from keybinds
+ Rename learntmux.markdown.html to tmux.markdown.html
```
Line  48: Added additional items to the list of keybinds for power users
Line 104: Created section on configuring ~/.tmux.conf & added additional comments
Line 203: Added section including external resources
```
it formally announces that the "a" key will be able to activate the prefix (C-a in this instance, now that it has been changed from C-b)

It should be under line 113 though, so I'll move it