doc/install.html: fix erroneous HTML annotation
align=middle is invalid; use align=center

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/106910044
docs: link to the assembler document from the Documents tab

Fixes issue 8156.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/108840045