doc: document version at which new language features were introduced in spec

doc: document version at which new language features were introduced in spec

Add a new section to the Appendix describing what features were
changed or added in which language version.

Add short links with references to the required language version
where relevant.

Fixes #63857.

Change-Id: I5250f856d8688a71602076fcc662aa678d96a5d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/549518
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Bypass: Robert Griesemer <gri@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>