Add Language Client Information into the main Sigstore documentation

[hayleycd]

Description
Sigstore users can use cosign for signing/verifying, but it is not their only option. The language clients do not have proper representation on the docs site. We should add information on the various language clients.

• sigstore-python
• sigstore-rs
• sigstore-js
• sigstore-java
• sigstore-go
• sigstore-ruby