doc: add classes to version tags in source install instructions
doc: add classes to version tags in source install instructions

This will allow us to mechanically substitute these strings
using javascript (in a forthcoming change to x/tools/godoc).

Updates #14371

Change-Id: I96e876283060ffbc9f3eabaf55d6b880685453e1
Reviewed-on: https://go-review.googlesource.com/22055
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

ImJasonH/go,CAFxX/go,akutz/go,nawawi/go,polyverse-security/go,golang/go,josharian/go,cloudflare/go,momchil-velikov/go,momchil-velikov/go,Cofyc/go,ganboing/go-esx,theif519/go,nawawi/go,nawawi/go,josharian/go,codestation/go,derekmarcotte/go,derekmarcotte/go,4ad/go,theif519/go,ganboing/go-esx,codestation/go,vsekhar/elastic-go,suncycheng/go,CAFxX/go,linux-on-ibm-z/go,momchil-velikov/go,cloudflare/go,suncycheng/go,derekmarcotte/go,4ad/go,vanloswang/go,vsekhar/elastic-go,akutz/go,ganboing/go-esx,ImJasonH/go,akutz/go,golang/go,linux-on-ibm-z/go,suncycheng/go,cloudflare/go,nawawi/go,4ad/go,derekmarcotte/go,ganboing/go-esx,4ad/go,CAFxX/go,suncycheng/go,linux-on-ibm-z/go,polyverse-security/go,vsekhar/elastic-go,4ad/go,cloudflare/go,josharian/go,goodlang/good,golang/go,goodlang/good,4ad/go,Cofyc/go,josharian/go,polyverse-security/go,vsekhar/elastic-go,polyverse-security/go,CAFxX/go,polyverse-security/go,golang/go,nawawi/go,suncycheng/go,Cofyc/go,vanloswang/go,ImJasonH/go,suncycheng/go,nawawi/go,vanloswang/go,nawawi/go,theif519/go,golang/go,CAFxX/go,goodlang/good,linux-on-ibm-z/go,polyverse-security/go,CAFxX/go,codestation/go,ImJasonH/go,linux-on-ibm-z/go,codestation/go,Cofyc/go,ganboing/go-esx,momchil-velikov/go,goodlang/good,momchil-velikov/go,suncycheng/go,akutz/go,Cofyc/go,goodlang/good,akutz/go,josharian/go,vanloswang/go,akutz/go,vanloswang/go,golang/go,codestation/go,josharian/go,derekmarcotte/go,theif519/go,Cofyc/go,golang/go,vsekhar/elastic-go,polyverse-security/go,4ad/go,akutz/go,linux-on-ibm-z/go,theif519/go,ImJasonH/go,goodlang/good,derekmarcotte/go,codestation/go,theif519/go,goodlang/good,josharian/go,4ad/go,suncycheng/go,CAFxX/go,josharian/go,nawawi/go,vanloswang/go,momchil-velikov/go,ganboing/go-esx,ImJasonH/go,ganboing/go-esx,cloudflare/go,vsekhar/elastic-go,polyverse-security/go,ImJasonH/go,linux-on-ibm-z/go,cloudflare/go,derekmarcotte/go,derekmarcotte/go,codestation/go,CAFxX/go,theif519/go,golang/go,goodlang/good,cloudflare/go,momchil-velikov/go,Cofyc/go,codestation/go,Cofyc/go,momchil-velikov/go,theif519/go,vsekhar/elastic-go,linux-on-ibm-z/go,vanloswang/go,vanloswang/go,akutz/go,christopher-henderson/Go,ImJasonH/go,vsekhar/elastic-go,cloudflare/go,ganboing/go-esx