Add SPIFFE auth plugin docs#1006

stevendpclark wants to merge 2 commits intovault/1.21.xhashicorp/web-unified-docs:vault/1.21.xfrom

stevendpclark_vault-38002_spiffe-docshashicorp/web-unified-docs:stevendpclark_vault-38002_spiffe-docsCopy head branch name to clipboard