[x/go.dev] content/about.md: add best practices section
[x/go.dev] content/about.md: add best practices section

Adds best practices section to be linked to from
the right sidebar on pkg.go.dev

Change-Id: I2ba4ab98c89c13257447f1306294838026a4cab4
X-GoDev-Commit: 0984db241fdfdb1d4e65c87b1afe63042a3a9c48

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website