[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
```
I see here you are closing off the markdown code block then below you are doing some markdown paragraphs. Our files usually go along the structure of having one giant code block that has nothing but code inside and anywhere that you want to discuss something, use a comment for that language to describe it. If you would do the same, that would be very kind. Just go ahead and use bash comments sense these are bash commands in this file.