Add intoto type documentation

[jspeed-meyers]

Summary

This PR proposes some documentation for the intoto pluggable type to help non-expert users understand fields not otherwise documented. I tried to keep this documentation proximate to the code, i.e. in the relevant GitHub section, and largely pointed to relevant specifications, rather than repeating at length information from those specifications. If this type of documentation is useful (and if it's not, no hurt feelings :) ), I'll submit similar documentation for other pluggable types as appropriate.

Related topic potentially better handled in separate issues and PRs:

• @nsmith5 pointed out that the rekor API returns a different format than the rekor-cli. This makes writing documentation harder. I did not address this aspect in the documentation I wrote. I figured that is a larger and separate topic.

Interested parties: @bobcallaway, @vaikas, @dlorenc, @SantiagoTorres, @znewman01

Ticket Link

Related to issue #665.

Release Note

NONE

[jspeed-meyers]

Sorry if I dorked this PR up while doing the DCO. Let me know and I will resubmit.