doc: PKI API table of contents (#2756)
doc: PKI API table of contents (#2756)

* Add a table of contents for api/secret/pki

* Fix the read certificate link

tallpauley/vault,MonsantoCo/vault,rnaveiras/vault,deverton/hashicorp-vault,lemondrank/vault,lirao/vault,mgk/vault,lirao/vault,joelthompson/vault,rnaveiras/vault,mgk/vault,mgk/vault,deverton/hashicorp-vault,hartsock/vault,lemondrank/vault,hartsock/vault,joelthompson/vault,kingland/vault,kingland/vault,rnaveiras/vault,joelthompson/vault,Aloomaio/vault,Caiyeon/vault,quixoten/vault,joelthompson/vault,wjam/vault,hartsock/vault,naunga/vault,Aloomaio/vault,hashicorp/vault,kingland/vault,quixoten/vault,Aloomaio/vault,hashicorp/vault,naunga/vault,wjam/vault,quixoten/vault,quixoten/vault,rnaveiras/vault,hashicorp/vault,MonsantoCo/vault,Caiyeon/vault,hartsock/vault,hashicorp/vault,mgk/vault,kingland/vault,Aloomaio/vault,kingland/vault,tallpauley/vault,hartsock/vault,hashicorp/vault,hashicorp/vault,rnaveiras/vault