Add Venafi Firefly to the SPIFFE tools page on spiffe.io

[RiichardCC]

Hi there, can you please add Venafi Firefly as a tool that implements SPIFFE on this page:

https://spiffe.io/docs/latest/spiffe-about/overview/#which-tools-work-with-spiffe).

Venafi is about to announce support for SPIFFE to meet security team use cases for workload identity authentication in cloud-native environments. Firefly is the first CA solution engineered for workload identity issuance. svg and png logo files are attached.

More information on Firefly here: https://venafi.com/firefly/

@azdagron As requested, here is a SPIFFE demo with Firefly: https://youtu.be/H8IA-_kH6Iw?si=uHYp64Mrvl2heB3e0