Create fulcio-spec.md

[tracymiranda]

Move Fulcio spec from Google doc to Github

Summary

This is the first Sigstore spec doc being moved to github for public collaboration

Release Note

NONE

Documentation

This migrates the Google doc with the following modifications:

• Add title and summary
• Separate introduction (why) and overview (what) sections
• Add numbering to sections for ease of referencing sections
• Improve consistency and conciseness of some text

[pwelch]

Thanks for migrating this!

[tracymiranda]

Based on initial reviews I've added the OIDC link and fixed the spacing issues.
For feedback on the terminology I've opened up issues so we can get reviews on it from the original authors.

[tracymiranda]

Thanks for the review @haydentherapper, added in the fixes from @chinenyeokafor's review.