Dock a window to the left or right side of your monitor

I guess you talk about tilling. You can simply install gnome3 or another DE that handles tilling by default as default DE in trisquel doens't.
You must be able to install a windows tilling manager in lxde or gnome flashback, but I don't know how

I think he means what is called either 'window splitting' or 'window snapping'.

Install Compiz

sudo apt-get install compiz

Then download 'trisquel7-window-manager-fix.tar' attachment:

https://trisquel.info/en/forum/window-resizing-issue#comment-83660

Extract, copy everything from the extracted folder /usr/share/gnome-session/sessions to /usr/share/gnome-session/sessions on your hard disk; also copy xsessions to /usr/share/

Log out, choose Compiz and log in. Now you can grab&drag any window to the edge of the screen and it will snap. Or use the shortcut Ctrl+Super+L/R Arrow).