docs/getting_started: update ubuntu install instructions
docs/getting_started: update ubuntu install instructions

To include git 2.28 install instructions (see #5088).

Also update version tags (to push 27.x over 26.x).

hlissner/doom-emacs,hlissner/doom-emacs,hlissner/.emacs.d,hlissner/doom-emacs,hlissner/doom-emacs,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/doom-emacs,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/doom-emacs,hlissner/doom-emacs,hlissner/.emacs.d