x/website: Effective Go needs to have a preamble re: generics

[griesemer]

Effective Go needs to be updated with a preamble/disclaimer as it was not written with generics in mind.

cc: @robpike

[robpike]

It should also mention that is old and dated (and frozen) and has little about libraries and some other emergent best practices. A preamble is probably the right fit here.

[robpike]

Duplicate of #48274

[susugagalala]

The book "The Go Programming Language" needs to be updated.

[amnonbc]

Effective Go is an excellent resource and should be kept current.

[ianlancetaylor]

Not doing this per CL 380634.

[gopherbot]

Change https://golang.org/cl/380634 mentions this issue: content/doc: add an explanatory preamble to Effective Go