Move gitsign under tooling section

[haydentherapper]

          gitsign is a unique case, it's more like Cosign in that it's a tool built on top of an SDK to sign a specific format (commits, rather than containers/blobs like Cosign).

What do you think about an additional section for Sigstore tooling that is not Cosign? Something like:

• Cosign
  • Sign
  • Verify
  • Etc
• Client libraries
  • sigstore-python
  • sigstore-java
  • etc
• Sigstore tools
  • gitsign
  • model signing (https://github.com/sigstore/model-transparency)
  • In the future, more tools

If we were to ever refactor Cosign to be smaller in scope to only support container signing, it would then just move under Sigstore tooling, though I don't expect that to happen. But it would give us a category to expand over time as more use cases for Sigstore signing arise.

Originally posted by @haydentherapper in #323 (comment)

[haydentherapper]

cc @hayleycd

Also cc @mihaimaruseac, who could help with some model signing docs