[x/go.dev] content/about.md: describe source linking
[x/go.dev] content/about.md: describe source linking

Explain how pkg.go.dev links symbols to source code, and describe
how the user can add those links.

Change-Id: Ia3c60737c16042497a46f0c75a618f929a4a8e9c
X-GoDev-Commit: b763616ed0ca2ccbbd59ab3f9808c8a095ccf167

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