Reorganize the UCAN spec in different repos

[bdehaynin]

• Move what is currently ucan-wg/spec to ucan-wg/delegation
• Make ucan-wg/spec into a high level description of UCAN, point at other specs
• Break out the Revocation section to ucan-wg/revocation
• Move ucan-wg/invocation to ucan-wg/invocation-ipld
• Make ucan-wg/invocation into a high level invocation spec
• Create ucan-wg/invocation-jwt (basically a copypasta of /invocation-ipld but in JWT format)