x/telemetry,x/website: add go.dev/doc/telemetry

[findleyr]

We need canonical documentation for Go telemetry: what it is (+what it is not), why does it exist, how does it work, how do you use it, etc.

• telemetry.go.dev/privacy is canonical, but only discusses privacy related issues
• @rsc's blog posts give a thorough overview, but are lengthy, don't reflect the current status, and aren't living documents

We discussed, and decided to add a new page on go.dev to host this documentation: go.dev/doc/telemetry.